Make Messages and Functions be in the same namespace

[stasm]

Forbid all uppercase ASCII identifiers for Messages and effectively make Messages and Functions co-exist in the same namespace.

## BEFORE

# FUNC is a FunctionReference
one = {FUNC()}
# FUNC is a MessageReference
two = {FUNC.attr}

With a single namespace for both Messages and Functions, two becomes a syntax error.

## AFTER

# FUNC is a FunctionReference
one = {FUNC()}
# FUNC is still a FunctionReference → Syntax Error
# (FunctionReference is not a valid `ref` of an `AttributeExpression`.)
two = {FUNC.attr}

[Pike]

We got around this on our call, right? This is WONTFIX, IIRC?

As this request is still open, adding here what made me raise concerns on our call: Changing the range of valid message IDs at this point is hard to be confident about.

[stasm]

Yep, this is a wontfix.