Require stdgates to be included to use common gates

[maloleroy]

Is your feature request related to a problem? Please describe.

When entering the prompt executable, I can use common gates directly, for example

qubit a;
x a;

However gates like x, cx or p are not part of the QASM language itself, but are instead part of the standard library.

Describe the solution you'd like
Introduce a --pure mode handling imports more strictly than now.

• prompt would still work like it is now
• prompt --pure would be more strict
• • Fail on incorrect imports
• • Require common gates to be imported before use

Describe alternatives you've considered
Enforcing the --pure mode as the default or only one.