Introduce compilation methods for exponentiated operators

[ecpeterson]

We're looking to introduce exponentiated skew-Hermitians as a Quil primitive, on the premises that:

• Many QPU users (and physicists in particular) are experienced at expressing problems in this language.
• This lends itself more readily to a 'sparse' representation than DEFGATEs do, which the compiler ought to be able to exploit.
• It is the compiler's job (rather than pyQuil's job, as is done presently) to translate these to native Quil instructions.

We should research some of the most common kinds of Pauli sums appearing in practice and corresponding expansion techniques, and then we should implement them.

[stylewarning]

@ecpeterson can we call this one Fixed (TM)?

[ecpeterson]

Yes!