Previously, each unique compiler cfg attribute that appeared in the
codebase was hard coded and emitted to stdout at compile time. This
meant keeping the file up to date as different compiler cfg attributes
changed. It's inconsequential to emit a compiler version that's not
used, so this change just emits all possibilities to reduce the
maintenance burden of the build script.
This implements basic facilities to conditionally carry string inputs in
parse errors. This includes:
* `InputString` type that may carry the input and format it
* `parse_error_type!` macro creating a special type for parse errors
* `impl_parse` implementing parsing for various types as well as its
`serde`-supporting alternative
This adds a build script that automatically activates the cfg if the
version is sufficient. We don't make API changes here, just improve
debugging by enabling `track_caller`.