V0.23.0 release tag

This release is primarily clarifications and improvements in error handling
and reporting.

The REQUIRE and ERROR_IF macros were introduced into the pseudocude to
describe the action when an operators required conditions are not met.
See section 3.1 of the specification for details on the new validation helpers.

Operator functionality changes:

Removed IDENTITYN, PLACEHOLDER, and RELUN. RELUN can be implmented with the
existing CLAMP operator. IDENTITYN and PLACEHOLDER can be handled with
appropriate graph actions

INTDIV - The integer division operator was renamed from DIV to clarify that it only operates on integer values.

PAD - The PAD operator now takes a value to be used as the pad constant.

Miscellaneous other clarification and cleanups were made to text and pseudocode.