This PR fixes the location of the error emitted when the `rintro` and
`intro` tactics cannot introduce the requested number of binders.
This patch adds a few `withRef` wrappers to invocations of
`MVarId.intro` to fix error locations. Perhaps `MVarId.intro` should
take a syntax object to set the location itself in the future; however
there are a couple other call sites which would need non-trivial fixup.
Closes #5659.
