Also change some tests with newly relaxed indentation requirements, and remove an irrelevant test (since unindented close parens are now perfectly valid, the test is no longer useful).
As previously discovered with #4464, it's easy to accidentally mis-use the State value returned on the Err path.
There were mixed assumptions about what that State represents: (1) the State where the error occurred, or (2) the State at the beginning of the thing we were just parsing.
I fixed this up to always mean (2) - at which point we don't actually need to return the State at all - so it's impossible for further discrepency to creep in.
I also took the liberty to refactor a few more methods to be purely combinator-based, rather than calling `parse` directly.
This removes the need to explicitly pass thru min_indent when using the parser combinators.
My ultimate goal here is to evolve the current parser closer toward a purely combinator-based parser,
at which point we can more easily transition smoothly to a formal(ish) grammar, or expand the meanings of combinators
to include things like:
* Incremental (re)parsing
* Unified parsing and formatting code
* Better error recovery
* Using the main parser directly for syntax highlighting
