Pancake __add_with_carry___ has unfortunate parser interactions

The `AddCarry` primitive was introduced to Pancake in #1398, and given concrete syntax that looked like this:

x = \_\_add_with_carry\_\_(y,z,w);

Because it looks so much like a function call, its parsing implementation piggybacks on the parsing of functions like this: during PEG parsing, nothing special is done so it becomes an ordinary `CallNT` node. Then during ptree conversion, the conversion of `CallNT` nodes has a ~~hack~~ special case for function calls of the precise form above, where they get replaced with the `AddCarry` primitive.

But function calls are allowed in many positions where `\_\_add_with_carry\_\_` is not: as stand-alone calls `f(args)`, as tail calls `return f(args)`, as calls with exception handlers, and so on. Ptree conversion needs to decide what do do when confronted with this. Prior to #1450, the behaviour in such cases was inconsistent: it would either fail during ptree conversion, or succeed and produce a `Call` to a function named `\_\_add_with_carry\_\_`. Both behaviours are bad: ptree conversion should never fail on the PEG parser's output, and it is confusing that nothing stops people from writing functions like this:

fun 1 \_\_add_with_carry\_\_(1 y, 1 w, 1 x) { ... }

...that is only callable on tuesdays.

In #1450 I made sure all out-of-place `__add_with_carry__` calls always fail ptree conversion, which is still bad, but at least consistent.

My proposed fix is to treat `__add_with_carry__` as a keyword during lexing, and handle it with its own kind of `NT` node in the PEG. Misusing the name would then produce (more) useful error messages.

Trace how the lexer, PEG parser, and ptree conversion handle the AddCarry primitive and out-of-place __add_with_carry__ calls. Start with the existing CallNT special case and the behavior described after #1450. Done means the name is handled as a keyword with its own PEG node, while misuse produces useful errors and ptree conversion does not fail unexpectedly.

Written by the indexing model from the issue text.