User-defined keywords

[pzinn]

This is very much work in progress, just to demonstrate how to have user-defined keywords: (cf #3593)

i1 : makeKeyword("×")

o1 = ×

o1 : Keyword

i2 : ZZ × ZZ := times

o2 = times

o2 : CompiledFunction

i3 : 1×2

o3 = 2

The most important change is to have a default operation (which applies to 99% of existing keywords, and to the newly created user-defined keywords) which is to look up the corresponding method. so no need to define all these identical functions one-by-one.
Right now only binary operations are properly created, I'm just doing a PR for the tests.

TODO:

• treat all the different cases (unary, binary, precedence) by adding options to the m2 level makeKeyword
• simplify drastically actors5.d by removing all unnecessary functions
• transfer the creation of keywords to m2 level (core)
• get rid of opsWithBinaryMethod etc
• get rid of getBinopName etc and rewrite pseudocode accordingly
• simplify the tex routines
• possibly remove entirely the fictitious Keyword class, instead providing some information in the Symbol Afterprint
• fix the other minor issues mentioned in User-created keywords #3593