Rust changes for v6.2

The first set of changes after the merge, the major ones being:

- String and formatting: new types `CString`, `CStr`, `BStr` and
  `Formatter`; new macros `c_str!`, `b_str!` and `fmt!`.

- Errors: the rest of the error codes from `errno-base.h`, as well as
  some `From` trait implementations for the `Error` type.

- Printing: the rest of the `pr_*!` levels and the continuation one
  `pr_cont!`, as well as a new sample.

- `alloc` crate: new constructors `try_with_capacity()` and
  `try_with_capacity_in()` for `RawVec` and `Vec`.

- Procedural macros: new macros `#[vtable]` and `concat_idents!`, as
  well as better ergonomics for `module!` users.

- Asserting: new macros `static_assert!`, `build_error!` and
  `build_assert!`, as well as a new crate `build_error` to support them.

- Vocabulary types: new types `Opaque` and `Either`.

- Debugging: new macro `dbg!`.
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