payload: Refactor as a concrete IndexTbl

Payload is now an index table, wherein each index is a byte string (or
compatible type).  The retrieval of indices will return a corresponding
offset or address of the indexed data (which is sensitive to the payload
base).  There is no longer a Symtbl member.

Due to this new design, the class no longer keeps a running payload
buffer that is appended to every time the payload is updated.  When the
user wants to get the full data, this buffer is constructed from the
Lict elements backing the payload.  This allows individual elements to
be modified or removed easily after they are inserted.

The use of a Lict allows data elements to be referred to by either their
positional array index, or the key specified when first creating that
element (done using the IndexTbl interface).

Payload objects may now be directly nested inside eachother, as opposed
to simply taking a payload's bytes and inserting those.  This allows
payloads to be used in a way resembling C structures.

The type-specific insertion functions have been removed and we instead
now lean on the __setindex__ interface inherited from IndexTbl to
directly assign values and append them to the payload.  In this case,
values are taken as-is from the assignment if they are bytes-like, and
automatically converted in some cases.

Payload's __call__ overload is now used to perform the quick, chainable,
and inline value insertion that was lost by the removal of the
type-specific functions.  "Calling" a payload with zero arguments will
still provide the old behavior of returning the payload bytes, however.

The semi-advanced features such as padding, alignment, and inserting
placeholder bytes have been removed from the main payload interface and
are now provided as compatible types that can be directly inserted into
Payload via the means described above.  In most cases, these are now
implemented to dynamically react to changes in the Payload content.  For
example, a "padlen" element, which is constructed with a fixed target
length parameter, will grow or shrink in length if the data preceding it
changes.

Automatic "badbytes" detection is removed, simply due to API conflict.
In my experience, this feature was little-used and can easily be done
manually by scripts if desired.  I don't plan to reintroduce this
feature.

pad_front functionality is also removed by this patch, since at the
moment it doesn't fit into the new design very well.  We may attempt to
reimplement it as a PayloadEntry down the road.  However, this feature
has also only seen rare use in my experience.

Signed-off-by: Malfurious <m@lfurio.us>

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