Check types of LHS and RHS in assignment "to identifier"

[jlapeyre]

Before there was no checking that the types of the LHS and RHS were valid in assignment, with the exception of checking constness.

This commit introduces some checking in the case that the LHS is represented by an identifier (i.e. not an indexed identifier) Checking if the LHS is an indexed identifier, for example b[1] = measure q; remains to be implemented.

• If types have same base array type, but dimensions differ, an IncompatibleDimensionError is recorded. This is tested, for example, with

bit[3] b;
qubit[2] q;
b = measure q;

• If the RHS is an integer literal and the LHS is unsigned, then the sign of the literal is examined and the RHS is wrapped in Cast if the sign is positive. Otherwise CastError.

• If the type of the RHS can be cast to that of the left, the RHS is wrapped in Cast.

• Otherwise IncompatibleTypes is recorded.

Several tests of these new behaviors are included.

A few details of things added:

• Fix erroneous Type of integer literal. Before we had UInt even though in the data we stored the sign of the literal. The type is now Int.
• Introduce semantic errors: IncompatibleDimensionError and CastError. The latter is for trying to cast a negative integer literal to an unsigned type. We could perhaps to do better. Perhaps just make this an IncompatibleTypes error instead.