Support for specifying the code model

The default code model is usually unsuitable for kernels,
so we add an option to specify which model we want.
mk: Fix LD_LIBRARY_PATH_ENV_NAMES in cross build
obsolete.rs: fix typo in message
rustc.1: fix typo

Uses the same wording as ``config.rs`` now.
complement-lang-faq.md: fix typos
debuginfo: Don't crash when encountering global variable with unknown source span.
Examples for Integer trait methods.
Main example for bigint usage.

Also use `///` for documentation instead of `/**`. End comments with
`.`.

Show imports for bigint example.

They might be useful.
Improve docs on JSON.

- add one simple example of using `ToJson` trait
- make example code more readable
stability lint: ignore code from macro expansion

This small patch causes the stability lint to bail out when traversing
any AST produced via a macro expansion. Ultimately, we would like to
lint the contents of the macro at the place where the macro is defined,
but regardless we should not be linting it at the use site.

Closes #15703
btree: use pattern matching instead of unwrap
syntax: Add ToTokens for Option<T>
syntax: Add ToTokens for Attribute_
syntax: Generalize ToTokens impl for Vec<T>

It will now `flat_map` over the elements of a `Vec<T>` if
`T: ToTokens`
Disallow importing from types when reexport is involved
syntax: Add quote_method!
lint: add method to get level of a specific lint.

This allows lint traversals to emit more information (when a lint is
non-allow), or avoid doing expensive computations (when a lint is
allow).
auto merge of #15698 : Zoxc/rust/code-model, r=alexcrichton

The default code model is usually unsuitable for kernels,
so we add an option to specify which model we want.

Testing for this would be fragile and very architecture specific and is better left to LLVM.
syntax: Add ToTokens impl for Method
auto merge of #15675 : errordeveloper/rust/json_docs, r=steveklabnik

- add one simple example of using `ToJson` trait
- make example code more readable