Implemented LET and LET* by variable renaming, which provides the

Implemented LET and LET* by variable renaming, which provides the
correct scoping semantics, and simplifies both the producing and the
produced code.

Removed the "." and "[]" name-mangling conventions from
symbol-to-js-string. Any code that uses symbols such as "foo.bar[baz]"
will now issue a warning, and needs to be rewritten to use standard
Lisp accessors. This is needed for variable renaming to work, and is
an extension of the patch that eliminated the ".method" method-calling
convention.

Thanks to Daniel Gackle and Dough Hoyte for opening my eyes to this
technique, which was right in front of me all along.