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:
```sh
#! /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. */