Swap order of argument for `environment-variables-get'
and `environment-variables-set!', so that the environment
variables come first --- which follows the usual order.
This change means that the parameter isn't used to get
the default environment variables, but that seems ok; the
convenient interface is `getenv' and `putenv'.
On Windows, case-normalized environment variable names.
Also, change the implementation to use an immutable hash
internally.
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 "=".
