rust-config: add support for default config items
Now that configitems.toml exists, we can read from it the default values for
all core config items.
We will add the devel-warning for use of undeclared config items in a later
patch when we're done adding the missing entries for `rhg`.
#require emacs
$ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
> -f ert-run-tests-batch-and-exit
Running 1 tests (*) (glob)
passed 1/1 hg-test-mode--compilation-mode-support* (glob)
Ran 1 tests, 1 results as expected* (glob)