Added symbols set to simplify logic for checking whether a variable name is shadowed by block instance variables or methods.
Added symbols set to simplify logic for checking whether a variable name
is shadowed by block instance variables or methods.

Allocate tag IDs for constant blocks separately from regular tag IDs so as
not to waste tag IDs which are limited by the 16-bit tag field. The
constant tag ID can be stored in the data field instead which would
otherwise go unused.

shaurz/ome,shaurz/ome