Support gate declaration in ASG (in addition to existing gate definition)

[jlapeyre]

The ASG supports gate definitions (or combined declaration/definition) but not declarations alone. Neither does the language spec mention extern for gates. So there is no reason to support at present parsing extern for gates in source code.

However, they would be useful in the ASG regardless of how they get there. Suppose we choose to understand gate rz(t) q {} to be a gate declaration. In the ASG as it is now, we enter t and q in the symbol table and store each in a vector in the ASG along with an empty vector for the body. But in fact all we need to store is a symbol for h and that the gate takes one angle parameter and one qubit parameter, which is much lighter weight.

• Find a way to handle externally defined gates #20
• Support extern gate declarations  openqasm/openqasm#507