Replaced all (non-test) instances of env!("HOME") with env::var("HOME").
The env! macro should only be used in instances where the environment
variable should be resolved during compile time.

Add the 'unset' command to remove global environment variable
definitions from the shell.

The shell now has support for aliases (via alias foo=bar). The 'unalias'
command is also available to remove aliases. Finally,
Environment::aliases was changed to be a HashMap<String, String>,
instead of a Vec<String>.

Since the verse's verb might change (for instance, it is an environment
variable, or an alias), add another check in Poem::recite, which simply
continues, instead of running the spellchecker, if the verb is empty.

Instead of having to pass around a bunch of different data structures
for various shell functions, create the wrapper compose::Environment,
which serves as a global shell state. It is configured via
login/profile/rc scripts initially, but can of course be modified
throughout the lifetime of the shell.

Use $PATH, instead of a hard-coded PATH from main(). This means that
there is no longer a need to pass around PATH to
repl()/recite()/path::refresh(), since path::refresh() can call env::var
directly.

Since the hard-coded paths were removed, there needs to be some way to
define $PATH. When running the debug build, dwvsh will look in
'dist/etc/dwvshrc' for the initial environment setup. For the release
target, dwvsh will look in '/etc/dwvshrc'. After the global rc file is
sourced, dwvsh will try to source ~/.dwvshrc if it exists, so users can
extend their environment without root access (assuming a release install).

The anthology module was added to run built-in commands. The 'cd' and
'exit' built-ins were moved from the main recite() loop to this module.
Additionally, the 'export' and 'source' built-ins were added.