Bump version to 0.2.55

This will bring in support for redox as part of the unix target_family, thus allowing https://github.com/rust-lang/rust/pull/60547 to move forward

Update to the latest wasi-sysroot.

 - Rename `wasm32-unknown-wasi` to `wasm32-wasi`.
 - `__wasilibc_rmfileat` was renamed to `__wasilibc_unlinkat`
 - Add bindings for a few more functions and typedefs.

Add futex constants

Adding some Futex constants. To be used in `parking_lot`. See https://github.com/faern/parking_lot/pull/1/files#r283110797

 - Rename `wasm32-unknown-wasi` to `wasm32-wasi`.
 - `__wasilibc_rmfileat` was renamed to `__wasilibc_unlinkat`
 - Add bindings for a few more functions and typedefs.

Improve newlib constants

This is necessary to set socket options on the Nintendo Switch, which uses a socket implementation directly ported from FreeBSD. Reopened from #1334 because I accidentally closed that.

redox: convert to target_family unix

This is the first step to supporting https://github.com/rust-lang/rust/issues/60139.

In order to have a smooth transition, there will need to be a change made in rust at the same time, switching Redox over to the unix target family. See https://github.com/rust-lang/rust/pull/60547

Update CI from ubuntu 16.04 (EOL) to 19.04

Closes #1333 .

See https://github.com/rust-lang/rust/pull/60775 .