Sort the output of the built-in 'export' and 'alias' commands, in cases
where all environment variables/aliases are printed out.

Previously, the recite() function created the 'out' variable, which was
a String, that got passed to the various incant functions, in order to
capture STDOUT in certain situations. In cases where STDOUT was
captured, it was first converted to a String, and then appended to the
'out' variable, by means of String::from_utf8_lossy(). This works for
basic text, however, does NOT work for binary data. This becomes
problematic, when for example, downling a tar file with curl/wget, that
is then piped ('|') to the tar program. Using from_utf8_lossy() in this
case can corrupt the tar file. This patch makes it so that out is stored
as bytes by default, and only converted to a String when necessary.

Instead of handling aliases in the recite() function, which requires two
loops to handle properly with the current implementation, offload
checking for aliases to the read() function.

Make sure to interpret alias values as their own poems, since aliases
can be fairly complex.

Add docstring comments for all the incant function throughout the
anthology, documenting what each function does, and an example of it's
shell command.

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).