Previously, if an environment variable was given in a string, such as:
'$PATH' or "$PATH", both would be replaced with the value of $PATH from
the environment. However, for single quotes strings, any values inside
should be taken at face value. In other words, echo '$PATH' should
simply write out: $PATH, and not whatever environment value $PATH
happens to be. This patch replaced '$' in single quoted strings with
the ASCII shift out (x0e) character. Any ASCII SO placeholder is
replaced with a '$' in recite(), after the environment variables have
been accounted for.

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 some better comments/move around existing comments to make some of
the actions in recite() more clear.

This patch fixes a bug for internal poems, where the output would always
get split on newlines, regardless of the verse's verb. This breaks
things like 'export DIR=`ls`', since output from `ls` might have
newlines, meaning that $DIR will only contains the first item from the
`ls` command. This will only perform the newline split when sending the
output to a non-builtin command.

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.

Need to include line to import compose::Environment.

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.