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

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.

This patch overhauls the reading and reciting of verses, such that the
redirection of STDERR (in addition to STDOUT, which was already a
feature), is now possible.

Removed the 'stdout' argument from recite(), since it is no longer
needed with how incantations function.

A verse's couplet indicator is now a u8, instead of a bool, with certain
values corresponding to types of couplets, for instance:

  ls | grep Ca | lolcat
  ^    ^         ^
  |    |         2: right side of a couplet
  |    3: both sides of a couplet
  1: left side of a couplet

Incantions are no longer hanlded in rune.rs, and the task macros have
been removed. Now, a verse incants itself, matching on its own meter to
determine how to handle the next verse.

The following runes were added to help with handling STDERR:
  Write2 -> 2>
  WriteAll -> &>
  Addendum2 -> 2>>
  AddendumAll -> &>>

The 'io' field in verse was changed from an Option<Rune>, to an array of
Runes, since a single verse might have multiple IO operations.

The following fields were added to Verse, to assist with handling
STDERR:
  ip -> List of filenames to read into STDIN
  op -> List of filenames to send STDOUT to
  ep -> List of filenames to send STDERR to

Keep track of channels when reading a poem. Channels are relating to IO
operations. If channel is None, words get pushed to the verse's primary
stanza (i.e. the verb or the clause). If a channel is selected, words
are pushed to one of the aforementioned new fields in Verse.
  Read -> ip
  Write/Addedum -> op
  Write2/Addedum2 -> ep
  WriteAll/AddendumAll -> op and ep

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.