Snippet parser for new engine

[immerrr]

This is an alpha version of parser discussed in #381.

It does pass some basic tests, but is by no means finished:

• it misses proper primary field determination
• it doesn't try combining field default values and transformations
• empty fields/mirrors result in (&field NAME nil) rather than (&field NAME)
• field names are left as strings
• almost no validation is done
• it may benefit better error position reporting

I'm afraid I won't have much time to hack on it in nearest future, so I figured I'd clean it up a bit and put it out as is.

[joaotavora]

I still haven't had a look at this, sorry, I will try by the end of the month when I resume work on the snippet-engine branch, which isn't making it into the next emacs pretest anyway...

[joaotavora]

Looks pretty at first sight, though...

[immerrr]

Hey there, has there been any progress with the rewrite?

[joaotavora]

Nope, sorry. I have a lot on my plate right now, but it has not been abandoned. The rewrite will hopefully be in the upcoming emacs version's core (not the 24.4, but the 24.5). @immerrr, if you are interested in tackling it yourself, I can put you up do date with information on the proposed changes by the Emacs maintainers with whom I was coordinating, send me a mail off-list.

[immerrr]

Nope, sorry. I have a lot on my plate right now

Ah, don't worry, all the same here and I'm not going to be of much help either in nearest future.