plai/{gc2/}mutator: doc fixes

A symptom of mismatched `for-label' imports was that searching for "<="
would show red-underlined links to the `plai/{gc2/}mutator' documentation.

Merge to v5.3.4
new environment-variable API

The `current-environment-variables' parameter determines the current
mutable "environment variable set". If that set is the initial one for
a Racket process, then using the set corresponds to working with OS
environment variables.  Otherwise, it's really just a hash table that
is packaged up as OS environment variables if a subprocess is created.

The new environment-variable interface works in terms of bytes, instead
of assuming that environment variable names and values fit in a string
encoding.

The string-based `getenv' and `putenv' are still available as
convenience wrappers. The checking on environment-variable names
for those wrappers is a little tighter, preventing any attempt to use a
name that contains "=".
racket/sandbox: add sandbox-make-environment-variables

By default, a sandbox gets a fresh environment variable set,
which means that it does not affect environment variables
outside the sandbox (which means that sandboxed code cannot
set the Racket process's OS-level environment variables).

Closes PR 13667
fix `dump-memory-stats' output

PTIdPTR use was wrong.