Rework key-mapping parsing.

There are two changes here:

1. Treat built-in key mappings and custom key mappings in the same way;
that is, we prepend the built-in mappings to the custom mappings and
then parse them all together.  This results in a number of
simplifications due to previous duplication of logic and the elimination
of special cases.

2. Parse key mappings in reverse order, so we can just ignore key
mappings after first encountering a key sequence.  So, `map`, `unmap`
and `unmapAll` are all treated in more or less the same way.

This is preparatory to reworking some aspects of the help page.  In
particular, regardless of the order of maps, unmaps, etc., this approach
makes it easier to find the mapping in effect for a key sequence (and be
able to recreate the order of those mappings).

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