Add the ability to increase instance search depth

[MatthewDaggitt]

Some of the tensor constraints about to be added require a deeper instance search than is currently available. This PR therefore:

• stores constraints solved by standard instance search separately from constraints solved by non-standard solvers (e.g. size solvers for Index constraints, polarity, linearity constraints)
• implements a depth counter for instance search and recursively calls it until the depth counter is exceeded. I hopefully on envisage ever needed a depth of 1.
• increases the depth counter to 1 only if we can make no further progress at depth 0.

No performance regressions in type-checking detected.