WyIL: Continued updates to support new CodeBlock API.

Various changes related to the continuing work to update the CodeBlock
API.  This includes changes to the binary representation of loop
bytecodes in WyIL files.  Also, the removal of list and set
comprehensions to further simplify the language and make this task more
accessible.  Finally, destructuring of loops is also currently disabled
and will need to be put back.
WyC: Removed SetComprehension tests

Removed all tests which were specifically for set comprehensions and
also updated other test cases which used set comprehensions.  Now, they
just use forall loops.
WyIL: Further Updates to CodeBlock API.

This represents a large number of continued changes focused on working
through the change to the CodeBlock API.  Specifically, that bytecodes
support true code blocks.
WyC,WyIL: Removed notion of try/catch blocks.

Removed syntax and implementation for try/catch blocks as part of my
efforts to simplify the language.  Try/catch blocks may come back into
the language later, though they may not be supported in the bytecode.
WyIL: Updates to AttributedCodeBlock.

This update sees progress getting attributed code blocks to work.  These
are code blocks which support attribution of elements.  They are used
for all outermost code blocks found in functions, methods, types, etc.

In this fix, SourceLocationMaps are also now functioning, though not
completed.
WyRT: Removed use of Exceptions.

Since exceptions have been removed for now, all uses of them must be
eliminated from the standard library as well.  Therefore, instead of
throwing an exception, such functions simply return null.