signatureHelp for Contructors too

Would be so nice, especially in pattern contexts.

also want to make the signatureHelp more robust (if type inference hasn't gotten that far, for example)

Start by locating the signatureHelp implementation and tracing how it handles constructors, pattern contexts, and cases where type inference has not reached the relevant expression. Done means constructor signatures are available in pattern contexts and signatureHelp remains robust when inference is incomplete.

Written by the indexing model from the issue text.