Add grune-json skeleton
Fixup prototype
Implement symbol to/from json
Add non_terminal serialization
Use ExternalProject_Add to pull in json11
Use ExternalProject_Add for cpp-flatten
Clean up encode() signature.
Require explicit symbol_traits instantiation.
Don't allow implicit conversions from T to symbol unless symbol_traits<T> exists.
Require an explicit RHS in productions.
Merge branch 'master' into json
Forbid symbol construction from certain list types.
Don't define symbol traits for all templates.
Merge branch 'master' into json
Remove symbol_traits for non-symbols.
Remove no-longer needed ctors.
Merge branch 'master' into json
Switch to using updated json11 API
Refactor json tests
Fixup json in unit tests