Provide instruction-based semantics for parameter arithmetic

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This refers to interpreting gate parameters as actual sequences of Quil instructions. For example,

RX(2*a) 0

is presently not very meaningful as a linear list of instructions. It could be interpreted as something like

temp = a
temp *= 2
RX(temp) 0

but Quil doesn't say.

[stevenheidel]

It would be neat to also allow something like: MOVE a 2*pi*b