Smaller optimizations for custom parser since before incorrect force push

[lue-bird]

starts at #256 without the incorrect rebase into force-push.

This branch rebased, accepting all commits until git merge-base smaller-optimizations-for-custom-parser-since-before-incorrect-force-push upstream/master (which is a06ca8a68744204edd00d38de4c29970dfa85b44),
followed by a git rebase --onto upstream/master <the merge base> and finally a force-push.

In case this overwrites some changes you've done since or is otherwise problematic, please comment.

Before, I tried rebase -Xtheirs and rebase -X ours but these seem to have included commits from upstream/master that messed up future history from my branch.

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actual PR content

• before: 7583.10000000149 ms to do 100 runs => 75.8310000000149 ms/run
• after: 2761.39999999851 ms to do 100 runs => 27.613999999985097 ms/run

(my old laptop broke down, so both of these are from my new one)

One change I really like is that now e.g. (\a -> { x = 3 }).x doesn't parse.