[#712] Deprecate morley language extensions

TODO:

• Update docs and README to clearly state that Morley language extensions are deprecated.

Description

Problem: Morley language extensions are barely used anywhere, and duplicate functionality from newer Michelson revisions. We'd like to stop spending effort on supporting them.

Solutions: Deprecate constructors corresponding to Morley extensions. Hide parsing mtz code behind a flag. One concession is that Morley parser is still more lenient, allowing lowercase commands, titlecase types, etc, hiding that behind a flag would require more effort than it's worth.

Related issue(s)

First half of #712 (closed)

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