In the interest of slimming down Doom's core (as we near v3), I've
deprecated these macros. They doesn't really need to exist. Sure, the
alternatives aren't as ergonomic or elegant, but they're good enough
that we don't need these trivial wrappers. Their local uses have been
refactored out as well.
`letenv!` is a layover from the days before
`with-environment-variables` (introduced in 28.x), and it remained
afterwards because I preferred the shorter name. From v3 and onward,
Doom's core will be put on a diet which, among other things, will
include culling redundant or superfluous functions/macros like this one.
BREAKING CHANGE: This restructures Doom's core in an effort to slim it
down and partially mirror architectural changes coming in v3. This is
part 2 of 3 commits (part 1 being 1590434), done to facilitate a change
in part 3 that will introduce a new `doom!` syntax for pulling
third-party module libraries from remote sources (similar to `package!`
statements). I am backporting this from V3 so I can move our modules out
into separate repos sooner than later, so development on modules can
continue separately without interfering with v3's roll out.
Though this is labeled a breaking change, it shouldn't affect most users
except those few tinkering directly with Doom's internals.
Ref: 15904349cf
