[CQT-217] Error when redeclaring a variable

[rturrado]

Up until now, a variable could be redeclared.

Upon a redeclaration, we would simply:

• remove the previous entry in the variable symbol table, and
• add the new variable to the symbol table.

But this was leading us to different kind of problems.

From now on, a program as the one below will emit an error message: trying to redeclare variable 'q'.

version 3.0

qubit[2] q

H q[0]

qubit[3] q

H q[2]

[elenbaasc]

Perfect, thanks!