Use bindgen on docs.rs

[yutannihilation]

Currently, the build on docs.rs fails with the following error (ref: build log). This is because the precomputed bindings for the version of R that docs.rs uses, R 3.6, doesn't contain the graphics-related objects because the file was generated long ago.

We have several options to avoid this error (see #415 (comment)), but I believe this is the best; we cannot expect what version of R docs.rs will use, but using bindgen should make the build success with arbitrary version.

[INFO] [stderr]  Documenting extendr-api v0.3.1 (/opt/rustwide/workdir)
[INFO] [stderr] error[E0432]: unresolved imports `libR_sys::DevDesc`, `libR_sys::R_GE_gcontext`
[INFO] [stderr]   --> src/graphics/mod.rs:83:20
[INFO] [stderr]    |
[INFO] [stderr] 83 | pub use libR_sys::{DevDesc, R_GE_gcontext};
[INFO] [stderr]    |                    ^^^^^^^  ^^^^^^^^^^^^^ no `R_GE_gcontext` in the root
[INFO] [stderr]    |                    |
[INFO] [stderr]    |                    no `DevDesc` in the root
[INFO] [stderr] 
[INFO] [stderr] error[E0412]: cannot find type `DevDesc` in this scope
[INFO] [stderr]   --> src/graphics/device_driver.rs:82:32
[INFO] [stderr]    |
[INFO] [stderr] 82 |     fn activate(&mut self, dd: DevDesc) {}
[INFO] [stderr]    |                                ^^^^^^^ not found in this scope
[INFO] [stderr] 
[INFO] [stderr] error[E0412]: cannot find type `R_GE_gcontext` in this scope
...snip...

[Ilia-Kosenkov]

Is there anything else here that needs to be addressed? Or should we review and merge already?

[yutannihilation]

First of all, can we agree on this direction? As commented on #415, there are many possible solutions.

[Ilia-Kosenkov]

As we seemed to agree to use bindgen, let's merge this PR.

[yutannihilation]

Thanks!