Native pragma support

[notmgsk]

Description

Add native pragma support to pyQuil.

PyQuil currently allows one to either use Pragma or bare strings to create pragmas. Both leave a lot to be desired:

p = Program('PRAGMA INITIAL_REWIRING "NAIVE"',
            H(0),
            "PRAGMA PRESERVE_BLOCK",
            CNOT(0, 1),
            "PRAGMA END_PRESERVE_BLOCK",
)

p = Program(Pragma("INITIAL_REWIRING", ['"NAIVE"']), # yes really
            H(0),
            Pragma("PRESERVE_BLOCK"),
            CNOT(0, 1),
            Pragma("END_PRESERVE_BLOCK"),
)

They're clumsy and not self-documenting. By adding some extra sugar, we can make pragmas both easier to discover and use:

p = Program(pragmas.NAIVE_REWIRING,
            H(0),
            pragmas.PreserveBlock(CNOT(0, 1)),
)

Closes #661

• The above description motivates these changes.
• There is a unit test that covers these changes.
• All new and existing tests pass locally and on Travis CI.
• Parameters and return values have type hints with PEP 484 syntax.
• Functions and classes have useful Sphinx-style docstrings.
• All code follows Black style and obeys flake8 conventions.
• (New Feature) The docs have been updated accordingly.
• (Bugfix) The associated issue is referenced above using auto-close keywords.
• The changelog is updated, including author and PR number (@username, gh-xxx).

[kalzoo]

A thought - what about making this an enum to group them together?

class InitialRewiring(Enum):
    Naive = Pragma("PRAGMA INITIAL_REWIRING \"NAIVE\"")
    ....

my_program = Program(
    pragmas.InitialRewiring.Naive,
    ...
)

Then the enum also becomes documentation of what the available options are.

[kalzoo]

Could this get tricky? Since it wraps the instructions, it might introduce cases where code that assumed program.instructions would be the whole program would break when it's only a single-element list, and in each place it will have to account for this.

Might be simpler to just vend the pragmas for block start and end separately?