Removes 'proto' dir

See #137 for the issue.

Every header in the proto directory was:

* Given include guards in the form

    #ifndef NEOVIM_FILENAME_H
    #define NEOVIM_FILENAME_H
        ...
    #endif /* NEOVIM_FILENAM_H */

* Renamed from *.pro -> *.h
* Moved from src/proto/ to src/

This would have caused conficts with some existing headers in src/;
rather than merge these conflicts now (which is a whole other can of
worms involving multiple and conditional inclusion), any header in src/
with a conflicting name was renamed from *.h -> *_defs.h (which may or
may not actually describe its purpose, the change is purely a
namespacing issue).

Once all of these changes were made a script was developed to determine
what #includes needed to be added to each source file to describe its
dependencies and allow it to compile; because the script is so short
and I'll just list it here:

    #! /bin/bash

    cd $(dirname $0)

    # Scrapes `make` output for provided error messages and outputs #includes
    # needed to resolve them.
    #   $1 : part of the clang error message between filename and identifier
    list_missing_includes() {
        for file_missing_pair in $(CC=clang make 2>&1 >/dev/null | sed -n "s/\/\(.*\.[hc]\).*$1.*'\(.*\)'.*/\1:\2/p"); do
            fields=(${file_missing_pair//:/ })
            source_file=${fields[0]}
            missing_func=${fields[1]}
            # Try to find the declaration of the missing function.
            echo $(basename $source_file) \
                \#include \"$(grep -r "\b$missing_func __ARGS" | sed -n "s/.*\/\(.*\)\:.*/\1/p")\"
        # Remove duplicates
        done | sort | uniq
    }

    echo "Finding missing function prototypes..."
    list_missing_includes "implicit declaration of function"
    echo "Finding missing identifier declarations..."
    list_missing_includes "use of undeclared identifier"

Each list of required headers was added by hand in the following format:

    #include "vim.h"
    #include "*_defs.h"
    #include "filename.h"
    /* All other includes in same module here, in alphabetical order. */
    /* All includes from other modules (e.g. "os/*.h") here in alphabetical
     * order. */

src/fold.h

@@ -0,0 +1,53 @@
+#ifndef NEOVIM_FOLD_H
+#define NEOVIM_FOLD_H
+/* fold.c */
+void copyFoldingState __ARGS((win_T *wp_from, win_T *wp_to));
+int hasAnyFolding __ARGS((win_T *win));
+int hasFolding __ARGS((linenr_T lnum, linenr_T *firstp, linenr_T *lastp));
+int hasFoldingWin __ARGS((win_T *win, linenr_T lnum, linenr_T *firstp,
+                          linenr_T *lastp, int cache,
+                          foldinfo_T *infop));
+int foldLevel __ARGS((linenr_T lnum));
+int lineFolded __ARGS((win_T *win, linenr_T lnum));
+long foldedCount __ARGS((win_T *win, linenr_T lnum, foldinfo_T *infop));
+int foldmethodIsManual __ARGS((win_T *wp));
+int foldmethodIsIndent __ARGS((win_T *wp));
+int foldmethodIsExpr __ARGS((win_T *wp));
+int foldmethodIsMarker __ARGS((win_T *wp));
+int foldmethodIsSyntax __ARGS((win_T *wp));
+int foldmethodIsDiff __ARGS((win_T *wp));
+void closeFold __ARGS((linenr_T lnum, long count));
+void closeFoldRecurse __ARGS((linenr_T lnum));
+void opFoldRange __ARGS((linenr_T first, linenr_T last, int opening,
+                         int recurse,
+                         int had_visual));
+void openFold __ARGS((linenr_T lnum, long count));
+void openFoldRecurse __ARGS((linenr_T lnum));
+void foldOpenCursor __ARGS((void));
+void newFoldLevel __ARGS((void));
+void foldCheckClose __ARGS((void));
+int foldManualAllowed __ARGS((int create));
+void foldCreate __ARGS((linenr_T start, linenr_T end));
+void deleteFold __ARGS((linenr_T start, linenr_T end, int recursive,
+                        int had_visual));
+void clearFolding __ARGS((win_T *win));
+void foldUpdate __ARGS((win_T *wp, linenr_T top, linenr_T bot));
+void foldUpdateAll __ARGS((win_T *win));
+int foldMoveTo __ARGS((int updown, int dir, long count));
+void foldInitWin __ARGS((win_T *new_win));
+int find_wl_entry __ARGS((win_T *win, linenr_T lnum));
+void foldAdjustVisual __ARGS((void));
+void foldAdjustCursor __ARGS((void));
+void cloneFoldGrowArray __ARGS((garray_T *from, garray_T *to));
+void deleteFoldRecurse __ARGS((garray_T *gap));
+void foldMarkAdjust __ARGS((win_T *wp, linenr_T line1, linenr_T line2,
+                            long amount,
+                            long amount_after));
+int getDeepestNesting __ARGS((void));
+char_u *get_foldtext __ARGS((win_T *wp, linenr_T lnum, linenr_T lnume,
+                             foldinfo_T *foldinfo,
+                             char_u *buf));
+void foldtext_cleanup __ARGS((char_u *str));
+int put_folds __ARGS((FILE *fd, win_T *wp));
+/* vim: set ft=c : */
+#endif /* NEOVIM_FOLD_H */