libR-sys 0.3.0

Low level bindings to the R programming language.
Documentation
# Changelog

## libR-sys (unreleased)

- Drop support for 32-bit Windows with R >= 4.2. As
  [the release note of R 4.1.0]https://stat.ethz.ch/pipermail/r-announce/2021/000670.html
  announced "the 4.1.x series will be the last to support 32-bit Windows,"
  there will be no 32-bit version of R as of R 4.2.0.  
  To be clear, libR-sys (and extendr) crate will keep supporting 32-bit on R <
  4.2 for a year or so.
- libR-sys no longer sets `DEP_R_R_VERSION_STRING` environmental variable.

## libR-sys 0.2.2

- Update the default precomuted bindings to R 4.1.0.
- Provide bindings for `R_ext/Altrep.h` and `R_ext/GraphicsEngine.h`.

## libR-sys 0.2.1

- Output R version info to downstream crates using variables `R_VERSION_MAJOR`,
 `R_VERSION_MINOR`, `R_VERSION_PATCH`, `R_VERSION_DEVEL`, and `R_VERSION_STRING`.

- Added precomputed bindings for Apple Silicon.

- Added contributing guidelines and code of conduct.

## libR-sys 0.2.0

- Provide precomputed bindings. Computations of bindings on the fly now only
  happens when the `use-bindgen` feature is enabled.

## libR-sys 0.1.10

- Minor fixes.

## libR-sys 0.1.9

- Remove need for pkg-config.