Avoid blow-up of complexity when squashing symbolic circuits

[cqc-alec]

Fixes #963 .

[yao-cqc]

from sympy import Symbol
from pytket.circuit import Circuit, OpType
from pytket.passes import SynthesiseTK
a = [Symbol(f"a_{i}") for i in range(100)]

c = Circuit(3)
for i in range(10):
    c.add_gate(OpType.CX, [], [0, 1])
    c.add_gate(OpType.Rz, [a[i]], [0])
    c.add_gate(OpType.CX, [], [1, 2])

SynthesiseTK().apply(c)

pytket 1.17 took 14.3s. The resulting circuit json is 2.7MB
this branch took 12.6s, but the expression is a lot simpler. 7kb

Not sure why it still takes that long to run.

[cqc-alec]

Not sure why it still takes that long to run.

Hmm, yeah I'm surprised it took so long...

[yao-cqc]

Thanks, I don't see any easy alternative to this...
Could you add this new argument to compiler_pass_v1.json?

[cqc-alec]

Could you add this new argument to compiler_pass_v1.json?

Done.