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/spell.h

@@ -0,0 +1,33 @@
+#ifndef NEOVIM_SPELL_H
+#define NEOVIM_SPELL_H
+/* spell.c */
+int spell_check __ARGS((win_T *wp, char_u *ptr, hlf_T *attrp, int *capcol,
+                        int docount));
+int spell_move_to __ARGS((win_T *wp, int dir, int allwords, int curline,
+                          hlf_T *attrp));
+void spell_cat_line __ARGS((char_u *buf, char_u *line, int maxlen));
+char_u *did_set_spelllang __ARGS((win_T *wp));
+void spell_delete_wordlist __ARGS((void));
+void spell_free_all __ARGS((void));
+void spell_reload __ARGS((void));
+int spell_check_msm __ARGS((void));
+void ex_mkspell __ARGS((exarg_T *eap));
+void ex_spell __ARGS((exarg_T *eap));
+void spell_add_word __ARGS((char_u *word, int len, int bad, int idx, int undo));
+void init_spell_chartab __ARGS((void));
+int spell_check_sps __ARGS((void));
+void spell_suggest __ARGS((int count));
+void ex_spellrepall __ARGS((exarg_T *eap));
+void spell_suggest_list __ARGS((garray_T *gap, char_u *word, int maxcount,
+                                int need_cap,
+                                int interactive));
+char_u *eval_soundfold __ARGS((char_u *word));
+void ex_spellinfo __ARGS((exarg_T *eap));
+void ex_spelldump __ARGS((exarg_T *eap));
+void spell_dump_compl __ARGS((char_u *pat, int ic, int *dir, int dumpflags_arg));
+char_u *spell_to_word_end __ARGS((char_u *start, win_T *win));
+int spell_word_start __ARGS((int startcol));
+void spell_expand_check_cap __ARGS((colnr_T col));
+int expand_spelling __ARGS((linenr_T lnum, char_u *pat, char_u ***matchp));
+/* vim: set ft=c : */
+#endif /* NEOVIM_SPELL_H */