Add haddocks for SharedTerm module

[brianhuffman]

Verifier.SAW.SharedTerm is our main user-facing api for building saw-core terms. We need to document it better.

For example, here's the documentation for scLambda:

scLambda :: SharedContext -> String -> Term -> Term -> IO Term

(I.e., there is none.) Users can't tell what the Term arguments are unless they look at the variable names in the source for hints.

We should have haddock documentation for every function exported from this module, at least.