metadata: improve some confusing type and module names

[jseyfried]

r? @eddyb

[eddyb]

Merge csearch.rs into cstore.rs.

As I've explained on IRC, this commit will create rebase issues (with at least my lazy-start branch).
The only thing I can think of that will likely keep the csearch.rs methods out of the diff (and prevent conflicts) is to use a third name for the resulting file (e.g. store.rs) - if cstore.rs is smaller, that is - and hope that git is clever enough to see a csearch.rs -> store.rs rename, with some extra bits added.

[eddyb]

@bors r+

[bors]

📌 Commit 80fe1d2 has been approved by eddyb

[bors]

⌛ Testing commit 80fe1d2 with merge 335c51b...

[bors]

💔 Test failed - auto-mac-cross-ios-opt

[jseyfried]

@bors r=eddyb

[bors]

📌 Commit f3993d1 has been approved by eddyb

[bors]

⌛ Testing commit f3993d1 with merge 4845add...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: eddyb
Pushing 4845add to master...