Add color to more keywords

[dlesbre]

This fixes #298
Adds missing syntax colors to:

• keywords Instance, Program Instance, Coercion, Context, Include, Search, Structure, Generalizable All, Generalizable Variables Existing Instance, Existing Class, Variant, About, Locate, Inline (if after Variable, Hypothese,...), Let Fixpoint, Let CoFixpoint, Canonical, the Obligation commands, some Proof commands (Save, Abort, Undo, Restart, Focus...), Implicit Types, Typeclasses Opaque, Typeclasses Transparent
• control-flow keyword fix ... for and where ... and.
• brackets {| and |}
• types : nat, bool, option, list, unit, sum, prod, comparison, Empty_set
• constructors : tt, true, false, Some, None, S, O, inl, inr, pair, cons, Eq, Lt, Gt, id, ex, all and unique.
• tactics : transitivity, etransitivity, f_equal, exfalso, replace, tauto, cycle, swap, revgoals, cycle, shelve, unshelve
• tactic keywords : do (move from tactic to keyword), tryif once, at, only
• snippets for Infix, match, Class/Record, Instance, Existing Instance, Generalizable ..., pose, destruct and replace
• fix Variable (x: type)., Context {a: type} and Context `{a:type} not coloring.
• fix Inductive > foo not coloring.
• Color identifiers when defining Class and Record similarly to Inductive

This includes all changes from #279.

[thery]

Look good to me. If no reaction, I will merge it tomorrow

[Blaisorblade]

Currently testing locally, but this seems a clear improvement worth merging :-)

[Blaisorblade]

FWIW, highlighting "built-in types" has effects like

and there was already some concern in #298 (comment). But on balance, this remains a net improvement that we can tweak later...

[thery]

@Blaisorblade can I merge?

[thery]

@dlesbre you are still doing some fixes I, great! Please tell me when you think it is ok and I will merge.

[dlesbre]

I've been adding missing stuff as I noticed it missing since I opened the PR. The last update were just somethings I noticed today. I think you can merge this now if you want too.

I'll open a new PR if I find enough missing things to warrant that.

[thery]

@dlesbre Perfect. Thanks a lot!