remove commented-out tests

`without-interruption` is deprecated anyway
minor refactor (DRY)
wip adding syntax-only prims under core.prim
wip redo let-handling code

all tests that handle correct code are passing.
some tests on incorrect code, fail.
wip reduce reliance on nvm.Instruction.syntax
wip rename nvm.Instruction.syntax to syntax2

to more easily distinguish it from core.Instruction.syntax
(such as when grepping)
wip store both core.Instruction and nvm.Instruction in AstNodes
wip get rid of prim._reference

just not needed anymore
wip minor cleanup
wip move ReferenceVisitor and AgentTypeChecker to the front end

we want AgentTypeChecker for Tortoise, and ReferenceVisitor
needs to run before AgentTypeChecker