Reformatting and starting personal edits of intermediate representation documentation.
Modified normalized IR description to match Normalized.asdl in the numba/numba asdl branch.
Proposed adding let to normalized AST.  Started proposal for untyped IR.
Updates to normalized and untyped IR's based on jriehl's proposal.
Re-remove comprehensions from untyped IR.
Attempting to bring typed and untyped IR into accord.