Refactor to search strategies

[Whebon]

Concrete structs can automatically inherit the fields of ProgramIterator with the @programiterator macro.

Todo:

• Update tests to work with the new iterators
• Refactor genetic_enumerators

[Whebon]

using Revise, HerbSearch, HerbGrammar, HerbData, HerbInterpret

g = @csgrammar begin
    Number = x | 1
    Number = Number + Number
    Number = Number * Number
end

problem = Problem([IOExample(Dict(:x => x), 2x+1) for x ∈ 1:5])

iterator = BFSIterator(g, :Number, max_depth=5)
solution, flag = synth(problem, iterator)

output = test_with_input(SymbolTable(g), solution, Dict(:x => 6))
println(output)