New repo: Grammar.jl

[sebdumancic]

should contain all code related to declaring a grammar and basic supporting functions (generating random expressions, getting number of rules and so on...)

Core functionality:

• specifying grammars: add, remove rules, read grammars from files, parsing macros, basic helper functions, store
  grammars
• core type: context-free grammar

Others:

• different types grammars: context-free, context-sensitive, probabilistic

Interfacing with other parts:

• no input dependencies from other packages
• Search.jl uses it to define iterators/search procedures

grammar = @cfgrammar begin
    Real = x                      # symbol
    Real = Real * Real            # julia expression
    Real = f(Real)                # function call
    Real = _(Base.rand(1.0:5.0))  # special syntax, eval argument of _() at derivation time
    Real = A | B | cos(Real)        # multiple rules on a single line
    Real = 1 | 2 | 3
    Real = |(4:6)                 # same as Real = 4 | 5 | 6
    Real = |([7,8,9])             # same as Real = 7 | 8 | 9 
end