Emit hole constraints in tcLhs to get rid of sig_wanted

Previously, we emitted hole constraints when type checking the type signature.
We emitted one hole constraint per wildcard occurrence (so multiple
constraints when a named wildcard occurs multiple times). Because these
constraints were emitted out of the scope of tcPolyInfer's captureConstraints,
we needed to capture and store them (in the sig_wanted field of TcSigInfo) to
add them later to the constraints generated when type-checking the body.

We now emit the hole constraints in tcLhs, which is within the scope of
tcPolyInfer's call to captureConstraints. This way, we no longer need to
separately capture and store these constraints. The ugly sig_wanted field in
TcSigInfo is thus removed.

A side-effect is that one constraint (and thus error message) is emitted per
named wildcard, not one per occurrence of the same named wildcard.
Add a field sig_partial :: Bool to TcSigInfo

Also reimplement isPartialSig which performed redundant checks, and remove
obsolete functions isWildcardTy and containsWildcards.
Refactoring to reuse checkConstraints
No insoluble constraint for an extra-constraints hole

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