Multi-rate method code uses subscripted variables

The code generated for the MRAB methods uses things like rk_y[2]. Indexing (and variables with structure in general) are not defined in leap's language--because they require extra (annoying) analysis to translate into static languages.

More specifcially, leap-lang's type system is right here:

https://gitlab.tiker.net/inducer/leap/blob/master/leap/vm/codegen/data.py#L36

It's daft, but it should suffice. :)

@mattwala, can you change the MR code so that it just uses flat variables? (Or is there something I'm missing that makes this impractical?)