refactor(options): reduce findoption() usage

Problem: Many places in the code use `findoption()` to access an option using its name, even if the option index is available. This is very slow because it requires looping through the options array over and over.

Solution: Use option index instead of name wherever possible. Also introduce an `OptIndex` enum which contains the index for every option as enum constants, this eliminates the need to pass static option names as strings.

test/functional/lua/ffi_spec.lua

@@ -38,7 +38,7 @@ describe('ffi.cdef', function()
           char *out,
           size_t outlen,
           char *fmt,
-          char *opt_name,
+          int opt_idx,
           int opt_scope,
           int fillchar,
           int maxwidth,
@@ -53,7 +53,7 @@ describe('ffi.cdef', function()
         ffi.new('char[1024]'),
         1024,
         ffi.cast('char*', 'StatusLineOfLength20'),
-        nil,
+        -1,
         0,
         0,
         0,