Throw error for boundary constraints using $rand against main trace

[grjte]

The use of $rand random values by boundary constraints should be limited to auxiliary constraints and therefore to boundary constraints which are against columns from the auxiliary trace.

We need to throw an error if $rand is used in boundary constraints defined against main trace columns. This should be handled in the IR.

For example, this should be an error:

...
trace_columns:
    main: [a, b]
...
boundary_constraints:
    enf b.first = a + $rand[0]
...

See:
#36 (comment)
#36 (comment)

[tohrnii]

Closed by #137.