Try out nightly renamed deps in Cargo.toml

[TheDan64]

Is your feature request related to a problem? Please describe.
The current version management story means we'd have to create separate crates for each llvm version, ie: inkwell-0.1.0-3_6, inkwell-0.1.0-7, etc. rust-lang/cargo#5653 might allow us to instead have a single crate that relies solely on the feature flags we already use today.

Describe the solution you'd like
For example this new cargo feature might allow us to instead do this

#[cfg(feature = "llvm3-6")]
extern crate llvm_sys_36 as llvm_sys;
#[cfg(feature = "llvm7-0")]
extern crate llvm_sys_70 as llvm_sys;

when Cargo.toml has:

[dependencies]
llvm_sys_36 = { package = "llvm-sys", version = "36.2.0" }
llvm_sys_70 = { package = "llvm-sys", version = "70.0.0" }

Describe possible drawbacks to your solution
None that I can think of; it would significantly improve the current approach for the end user (if it does indeed work this way).

[TheDan64]

...This doesn't seem to work due to conflicts in linking even when using conditional compilation:

error: failed to select a version for `llvm-sys`.
    ... required by package `inkwell v0.1.0 (/mnt/d/LinuxData/repos/inkwell)`
versions that meet the requirements `^50.2` are: 50.2.0

the package `llvm-sys` links to the native library `llvm`, but it conflicts with a previous package which links to `llvm` as well:
package `llvm-sys v40.2.0`
    ... which is depended on by `inkwell v0.1.0 (/mnt/d/LinuxData/repos/inkwell)`

failed to select a version for `llvm-sys` which could resolve this conflict

[TheDan64]

Unfortunately it doesn't seem like they'll support our use-case :(