Added annotated expr nodes.
Eval using annotated nodes. No change to Eval code

Added boilerplate for an initial "check" phase which
will detect errors that would be found at compile time
by a traditional compiler.
Whitespace
WIP checkBinaryExpr untested
WIP check UnaryExpr and BasicLit untested
WIP const evalutation code.
WIP Mixed type/untyped const expressions.

Middle of writing tests
WIP failing floating-point % test
WIP Typed ConstNumbers. floating % test passing.
WIP Const numeric binary op tests done.
WIP Added errors for numeric op (nil|bool|string)