Use enums to clarify `DepNodeColorMap` color marking
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 PR 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 colour for the node.
The updated docs for `try_set_color` also fix a typo: *atomicaly* → *atomically*.
r? nnethercote (or compiler)
Jonathan Brouwer
baffc5e9bc