AutoQASM: Add support for FreeParameter as gate arguments

[laurencap]

This allows statements such as rx(0, FreeParameter("theta")) in AutoQASM programs.

When converted to OpenQASM, the resulting program has an input statement, such as input float[32] theta;.

https://openqasm.com/language/directives.html#input-output

The input modifier can be used to indicate that one or more variable declarations represent values which will be provided at run-time, upon invocation. This allows the programmer to use the same compiled circuits which only differ in the values of certain parameters. For backward compatibility, OpenQASM3 does not require an input declaration to be provided. When an input declaration is provided, the compiler produces an executable that leaves these free parameters unspecified: a circuit run would take as input both the executable and some choice of the parameters.

input type for FreeParameter (input float32 a)

[laurencap]

Closed by #762
Additional work is required, but new issues have been made to track that work.