add AST building as beaver parser generator

[b3b00]

beaver provides a compact way to produce AST.
Provides CSLY a similar mechanism.
Maybe it needs an externalisation to a grammar file ( as beaver does), if so the grammar MUSTbe parsed with CSLY (eatsyourown dog food principle )
Must not add a build step to produce the parser as it s the leading CSLY feature.

[b3b00]

Not sûre this fracture has some real value. While and json samples demonstrate that an AST building parser is very compact with basic CSLY features