This is an extension of #2979 that provides an API function for calling dict functions.
Note that this branch is based on those changes since I could reuse that code completely for the implementation of the current state.

@ZyX-I proposed this:

Or, maybe a better idea: strip off self handling from vim_call_function completely (restore the original functionality), but additionally add vim_call_dict_function(Object self, Boolean internal, String functionName, Array args, Error *err) which will do the following:

1. Convert self to a dictionary: it should either be a Dictionary or a String object, in first case it is simply converted dictionary, in the last case it is expression passed to eval() which must evaluate to a dictionary.
2. If internal is true then check whether functionName is a key in self that refers to a function reference. If yes, replace function name with value of this reference. If no, raise an error. If internal is false then functionName is passed to call_func untouched.
3. Call call_func with arguments and self dictionary.

Intended usage: same as vim_call_func, but for constructs like g:SyntasticLoclist.current().filter(…): g:SyntasticLoclist.current() goes into self as a string, arguments of .filter() may be different and go into args, rationale is the same: first string is a simple static string that does not change, arguments in this case do not require double serializing (from dictionary expected by .filter() to string then string to messagepack).

Additional rationale (behind the internal argument): this make it possible to call anonymous functions without knowing their internal name (i.e. number), thus anonymous functions can be truly anonymous like in extended-funcref branch (they are not stored in a global functions dictionary under any name there). (Or in VimL-to-lua translator, if I remember correctly.)

Here is a list of things that still have to be discussed, tested or implemented (inclusive or):

• Calling with a string to be evaluated to a dict and internal set to false
• Calling with a string to be evaluated to a dict and internal set to true
• Calling with a msgpack-rpc dict and internal set to false
• Calling with a msgpack-rpc dict and internal set to true