Toward common subexpression elimination in subscripts: generate C (and related languages) via an AST

Some kernels require the duplication of an iname to be logically schedulable.
This commit introduces an algorithm that lists such options. It also provides
a function to check whether such transformation is needed.

Add for and with blocks to loopy-lang, for easier explicit iname decl, improve instruction parsing, adapt Rob's Bernstein tests

This reverts commit f8f65232.