Add support for token weights during fuzzing

[Elias Castegren (EliasC)]

This PR adds support for supplying token weights to the fuzzer. The weights are supplied as a map from tokens to integers and unmapped tokens defaults to 1. The driver CLI has been extended:

$ ./infix_trieste test --weights infix-int 2 infix-output 0
# prefer integers, exclude output statements

Note that setting a weight to zero may cause generation to abort with an exception if there is no viable option for a choice. I thought about checking for that up front (either checking for paths in the WF spec that lead to dead choices, or propagating dead choices upwards so that the path cannot happen), but I think the benefit doesn't outweigh the benefits in this case.

A possible future thing to add would be an automatic tuning of the weights based on the patterns of a pass: if the patterns have In(Foo) and T(Bar) that suggests that we should increase the weights of Foo and Bar. Preliminary experiments suggests that this reduces the number of trivial trees (trees that do not match a single rule).