Migrate to use unified grammar tables

Previously we keep a separate static grammar table for opcodes/
operands per SPIR-V version. This commit changes that to use a
single unified static grammar table for opcodes/operands.

This essentially changes how grammar facts are queried against
a certain target environment. There are only limited filtering
according to the desired target environment; a symbol is
considered as available as long as:

1. The target environment satisfies the minimal requirement of
   the symbol; or
2. There is at least one extension enabling this symbol.

Note that the second rule assumes the extension enabling the
symbol is indeed requested in the SPIR-V code; checking that
should be the validator's work.

Also fixed a few grammar related issues:
* Rounding mode capability requirements are moved to client APIs.
* Reserved symbols not available in any extension is no longer
  recognized by assembler.