Simplify fuzzer generation of function references

[tlively]

When creating a reference to func, fix the probability of choosing to
continue on to choose some function other than the last one rather than
making it depend on the number of functions. Then, do not eagerly pick
from the rest of the candidate functions. Instead, fall through to the
more general logic that will already pick a random candidate function.
Also move the logic for coming up with a concrete signature down to
where it is needed.

These simplifications will make it easier to update the code to handle
shared types.