When a function's documentation has to explain the meaning of nested results
and options, then it is often a good candidate for using a custom result enum
instead.
This commit also renames `DepNodeColorMap::try_mark` to `try_set_color`, to
make it more distinct from the similarly-named `DepGraph::try_mark_green`.
The difference is that `try_mark_green` is a higher-level operation that tries
to determine whether a node _can_ be marked green, whereas `try_set_color` is a
lower-level operation that actually records a color for the node.
`typos.toml` has an exception for "numer", to avoid flagging its use as
an abbreviation for "numerator". Remove the use of that abbrevation,
spelling out "numerator" instead, and remove the exception, so that typo
checks can find future instances of "numer" as a typo for "number".
Zalathar