Prepare to use rls-data 0.12

[Xanewok]

Use rls-data 0.12.
From what I understand, this is needed to so the extern branch of rls (multiple-analysis, includes passing analysis data via JSON) can be pulled for rust-lang/rust#45468, without also indirectly pulling my branch with #106, which seems to fail as rustbuild uses --frozen?

See also: rust-lang/rust#45468 (comment)

[Xanewok]

This essentially behaves the same as current master, only it uses data from rls-data 0.12.
Updated tests (also the ones for rls-data 0.11 update) and bumped the semver patch version.
Not sure if this is correct from semver pov, but it allows to still use 0.7 for now, until the compiler starts emitting data compatible with this version.