update version to 0.18.0

update version to 0.17.0

we now version all crates in lockstep

as they are developed together, they will never depend on different versions of each other

Add extra traits to syn

Building c2rust-ast-printer alone requires the "extra-traits" feature of
syn.

Last commit that includes cross checking functionality.

Add tests for packed unions

Update c2rust-asm-casts to 0.2.0

c2rust-asm-casts is now compatible with no_std

Bump version to 0.15.0

Bump c2rust version to 0.14.0 (#222)

* Update dependency patch versions

* Update crates.io README instructions for 0.14.0 release

* Bump c2rust version to 0.14

Add more fields to ast-printer Cargo.toml

[skip ci]

Bump to version 0.12