This makes it easier to populate unstable
options from configuration files. Might also
help make unstable option tests easier to write.

Obviously this only works with nightlies and all
that, but if you're e.g. testing resolver v2, want
to always have Ztiming on in your workspace, or
something like that, this makes it easier to do.

This makes it a little easier to match whatever
the natural formatting is for where you're setting
unstable options -- CLI or config file.

Co-authored-by: Eric Huss <eric@huss.org>

These masquerade like construction functions, but they're
in-place mutators since they're meant to be used one after
another in a precedence chain.

Per comments on the PR, a couple changes need to be backed out.

Tests are currently failing, looks like there's something in the
Deserializer impl that's forcing field-missing errors even when
the serde `default` annotation is applied.

This is a workaround in case y'all would like to land this change
before I'm able to finish up a Deserializer refactor.

The current behavior, with the default-false workaround in place,
is not able to identify extra members of the `unstable` table, so
it can't error on unexpected members.

I'll add this test back in with a Deserializer refactor to clean up
the extra logic added to CliUnstable.

This patch changes how ConfigMapAccess iterates k/v pairs when
deserializing structs.

Previously we produced keys for exactly the set of fields needed
for a struct, and errored out in the deserializer if we can't find
anything for that field.

This patch makes us produces keys from the union of two sets:

1. All fields that are both needed for the struct and can be found
   in the environment.
2. All fields in the config table.

This change allows serde's codegen to handle both missing and
unknown fields via the usual derive annotations (default or
deny_unknown_fields respectively)

Serde's errors for missing fields are a lil' different than
the ones from our Deserializer.

These tests demonstrate the current failure mode around
overlapping env keys and inner structs. To some extent this
is a limitation of mapping on to environment variables with
an underscore as both the namespace separator and the
substitute for dashes in flag names in that the mapping is
not strictly one-to-one.