[llvm] Add support for accessing ProGraML as JSON.

[ChrisCummins]

This adds a new ProgramlJson observation space that bypasses the
networkx graph construction and instead returns the raw node link data
for ProGraML graphs:

>>> import compiler_gym
>>> env = compiler_gym.make("llvm-v0")
>>> env.reset()
>>> env.observation.ProgramlJson()
...
{'block': 60,
 'features': {'full_text': ['store i32 %182, i32* %17, align 4, !tbaa !4']},
 'function': 13,
 'id': 995,
 'text': 'store',
 'type': 0},
{'block': 60,
 'features': {'full_text': ['i32 %182']},
 'function': 13,
 'id': 996,
 'text': 'i32',
 'type': 1},
{'block': 60,
 'features': {'full_text': ['i32* %17']},
 'function': 13,
 'id': 997,
 'text': 'i32*',
 'type': 1},
{'block': 60,
 'features': {'full_text': ['br label %183']},
 'function': 13,
 'id': 998,
 'text': 'br',
 'type': 0},
{'block': 61,
 'features': {'full_text': ['%184 = load i8*, i8** %9, align 8, !tbaa !0']},
 'function': 13,
 'id': 999,
 'text': 'load',
 'type': 0},
...]}

[bcui19]

LGTM