Add capability to annotate statements

[PhilReinhold]

This adds support for openqasm annotations

Annotations can be added in three ways:

1. Program.annotate which attaches the annotation to the next statement added to the program
2. _ClassicalVar(..., annotations=[...]), which attaches the annotation to the variable declaration statement
3. @annotate_subroutine(...) decorator, which attaches the annotation to the subroutine declaration

[jcjaskula-aws]

Minor comments so far. Believe we should discourage customers to use prog.annotate with prog.declare. I think the annotation is added to the first statement and not the others.

prog = oqpy.Program()

j = oqpy.IntVar(0, name="j", annotations=["annotation-j"])
i = oqpy.IntVar(0, name="i", annotations=["annotation-i"])
prog.annotate("first-annotation")
prog.declare([i,j])
print(prog.to_ast())

Should we defer the support for QubitDeclaration? Right now, it does not seem that we add the declaration anyway even without annotation.
Otherwise, it looks good to me.

[PhilReinhold]

Believe we should discourage customers to use prog.annotate with prog.declare. I think the annotation is added to the first statement and not the others.

I've added to the test to confirm that the annotations "work as expected" (at least how I would expect them to work) with mixing annotations between variable initialization and prog.annotate(); prog.declare().

Should we defer the support for QubitDeclaration? Right now, it does not seem that we add the declaration anyway even without annotation.

I've added code to make it work with qubit declaration as well in the same way.

[jcjaskula-aws]

LGTM.

I agree that annotate() + declare should annotate the first as the parser would if we pass @my_annotation + int i=0.