Fixed some issues when setting up new environment.

[sevenc-nanashi]

(My mother language is not English, so I may say some inappropriate things.)
I tried to contribute project, but there were some errors. I fixed them.

• The development of this project requires nightly rust (as test uses -Z). Added rust-toolchain.toml.
• If proc-macro2 is older than 1.0.60, the build will fail. (cf: Regression building proc-macro2 1.0.59 with nightly-2023-06-28 rust-lang/rust#113152 (comment)) Changed the dependency section.
• The tests fails with Cython 3. Added some notes in contributing.md.

Cython 3 detail

As cython 3 prints this warning, the snapshot doesn't match.

Error compiling Cython file:
------------------------------------------------------------
...
from libc.stdint cimport int8_t, int16_t,

int32_t, int64_t, intptr_t
^
------------------------------------------------------------

tests/expectations/workspace.tag.pyx:1:0: Dotted filenames ('workspace.tag.pyx') are deprecated. Please use the normal Python package directory layout.

[jschwe]

This project requires nightly rust (as test uses -Z). Added rust-toolchain.toml.

Some features / tests require nightly (expanding macros), but in general cbindgen runs fine on stable Rust 1.57 and newer

[sevenc-nanashi]

Oh, my writing was bad. Fixed