Improved comments on the constraint checking logic.
Automatically generated comments on many VDBE opcodes when in SQLITE_DEBUG
most.  Comments derive from the "Synopsis:" field added to each opcode
definition in vdbe.c.
Import the automatic comment generating changes from trunk.
Enhanced display of register ranges in the auxiliary comments added to EXPLAIN.
Add the SQLITE_ENABLE_EXPLAIN_COMMENTS compile-time option to enable extra
commentary in the EXPLAIN output.  Formerly, this was only available with
SQLITE_DEBUG.
Merge EXPLAIN enhancements from trunk.