chore(deps): update rust crate lalrpop-util to 0.21

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
lalrpop-util	dependencies	minor	0.20 -> 0.21

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Release Notes

lalrpop/lalrpop (lalrpop-util)

v0.21.0

Compare Source

Since the last release, a fair number of the commits have been focused on cleaning
up and improving LALRPOP's documentation. Shout out to Yudai Takada, George
White, and Dinu Blanovschi.

Features

• LALRPOP now throws an error in more cases where it would previously just write
  out an error message and exit.
• lalrpop::process_src is now the recommended function to use in build.rs
  files. Previously the documentation incorrectly suggested that
  lalrpop::process_root looked in ./src instead of .

Bugfixes

• A long-standing bug where LALRPOP would throw a "no entry found for key"
  exception when trying to handle certain grammars has been resolved.
• LALRPOP will stop expanding macros infinitely during build time via a new
  macro_expansion_limit.

Compatibility note

Adding a limit to the number of times that LALRPOP will attempt to expand a
macro is technically a breaking change. However, the default limit of 200
should be more than enough for the grammars we are currently aware of (which
almost always need a limit of less than 5). This limit is customizable via Configuration::set_macro_expansion_limit.

If you have a grammar that uses a significant amount of macro expansion steps,
we would be very interested in a PR that adds it to the test suite.

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