relocate StructureDeclarations to core

this is preparatory work towards creating core AST classes.
we'll be using StructureDeclarations.Procedure in those,
we anticipate.
wip split core.AstNode off from compile.AstNode
wip refactor so backifying of prims is its own subphase
wip make ExpressionParser use core ASTs up until the last minute
wip get backifier stuff out of ExpressionParser