experimental cps detection via scope injection

[alaviss]

This employs the use of a "phantom" scope to inject new symbols into the body of a cps block before they are semantically checked.

After that, we surgically extract the transformed output, lifting the result from the phantom scope to the invocation scope.

The experiment currently injects a single symbol: isInCps, which would allow macros/template to detect if they are in cps scope.

This stuff doesn't build yet, a simple:

proc foo() {.cps: Continuation.} = discard

fails with:

cps/cps.nim(245, 41) Error: inconsistent typing for reintroduced symbol 'continuation': previous type was: cps:foo() env; new type is: cps:foo() env

on NimSkull.