When using IO operations from within a file, the channel would get
overriden before pushing a filename to the appropriate vector (op, ep,
or both). This is because Rune::Continue is used for newlines, and was
resetting the channel to None, before the push operation. Rune::Continue
is now broken out into its own match clause, that doesn't set the
channel before calling verse.add().

The code to find and replace environment variables was only searching
for alphanumeric characters. This means that the shell was unable to
recognize environment variables that used an underscore, for instance.
This patch allows the underscore character to be used when settings and
reading environment variables.

This patch update a ton of the documentation comments throughout the
codebase, refactoring some areas, and adding new comments to others.

Sometime when the switch to the new built-in command system was
happening, we lost the logic to force the capture the output of STDOUT,
mainly used for running internal poems (i.e. 'ls `ls`'). This patch adds
a new field to the Environment struct, called fc (force capture). It
gets set to true before running internal poems, and unset afterwards.
Finally, some checks were added to the incant!() macro to properly
handle STDOUT when fc is set.

This commit reworks spellcheck() so it is more verbose about what it
returns. It also re-introduces the use of spellcheck() in
Poem::recite().

Spellcheck now returns a value in the enum, Spelling:
  FullPath -> Indicates a full path was specified as the verb
  OnPath -> Indicates the verb is on the $PATH
  BuiltIn -> Indicates the verb is a built-in command
  None (Option) -> The verb does not exist

This commit also removes some defunct (commented-out) code.

Previously, built-in commands were fairly primitive, merely outputting
STDOUT and STDERR with the print! macros. However, we need them to
behave like normal programs, that is:

  - Acknowledge their verse's meter (forking, piping, etc.),
  - Ability to capture STDOUT and STDERR (>, 2>),
  - and Affect the currently running environment.

For these reasons, the anthology was reworked, and now contains the
Anthology struct, which mimics both std::process::{Child, Command}.

The AnthologyStdin helper struct was also created, for built-ins to
take input on STDIN, though no built-in is currently using it.

Each built-ins' incant functions were updated to return a
std::process::Output. It contains output from STDOUT, output from
STDERR, and the exit code of the "process".

A fix was also implemented for aliases, where the STDOUT and STDERR
vectors were not being copied to the newly constructed verse.

Added a built-in which command (for MacOS and BSD), which can check
aliases, and other shell built-in commands, in addition to bins on the
$PATH.

Instead of returning true or false, if a given bin is found on the
$PATH, return the index of where the bin was found. This is useful, in
case we want to actually get the full path of the bin.

If a full or relative path is specified, the function will return the
length of the bins vector.

Added documentation comments for the Poem::recite() function, detailing
what the purpose of the function is, as well as what operations it
performs.

Added definitions of fields for the Verse struct, and updated its
wording to accomodate for the type change of the couplet field.

Updated the wording of Verse::add() to make it more verbose, and also
added descriptions of the arguments, alongside some example usage.

Added documentation comments for the Verse::incant() function.