retool MIR passes completely

The new setup is as follows. There is a pipeline of MIR passes that each
run **per def-id** to optimize a particular function. You are intended
to request MIR at whatever stage you need it. At the moment, there is
only one stage you can request:

- `optimized_mir(def_id)`

This yields the final product. Internally, it pulls the MIR for the
given def-id through a series of steps. Right now, these are still using
an "interned ref-cell" but they are intended to "steal" from one
another:

- `mir_build` -- performs the initial construction for local MIR
- `mir_pass_set` -- performs a suite of optimizations and transformations
- `mir_pass` -- an individual optimization within a suite

So, to construct the optimized MIR, we invoke:

    mir_pass_set((MIR_OPTIMIZED, def_id))

which will build up the final MIR.

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