[WIP] Refactor the definitions of RPC functions

[kermorgant]

Thanks to @ejmr for this pull request (he's the author of this proposal)

---

IMPORTANT: This code is unfinished and needs a lot more testing.
Developers should not build off this commit because it is highly
likely we will rebase it once we address all problems.

We define a number of functions whose sole purpose is to invoke
Phpactor's RPC commands. The majority of these functions are the same.
That is, their only real difference is the name of the RPC command and
what we pass to our phpactor--rpc function. This patch refactors
that code for multiple purposes.

1. We now only need to change one place to add support for a new
   command: the 'phpactor--rpc-command-data' list. The exception to
   this are the few RPC commands like "echo" which require additional
   arguments. It seemed easier to leave those few functions as they
   are instead of trying to shoe-horn support for a corner case into
   our now macro.

2. The new 'phpactor-defrpc' macro automatically creates the functions
   we previously wrote by hand. This both reduces bloat and decreases
   chances for errors, since now our code for such functions is all in
   one place: the macro definition itself.

3. We create a list of relevant data about the RPC commands which we
   can loop over, apply our new macro to each. This lets us introduce
   support for a new RPC command by modifying a single line in a list
   as opposed to writing an entire function. (Again, the exception are
   those few commands like "echo" which require additional arguments.)

[kermorgant]

I was going to give this another try but this time, it starts giving this type of error

phpactor.el:541:1:Error: Wrong type argument: stringp, (car rpc-data)

[ejmr]

I get the same issue. Maybe instead of a macro we replace it with a function-defining-function and then (mapcar) over all the relevant data in the defconst list. Just brainstorming aloud.