backend: initial support for matches on tuples

This is a first take on compiling matches on more complex expressions,
starting with tuples.

The general idea is to first rearrange the match into a simpler form, in
which only one element of the scrutinee tuple is examined at a time.
The remaining elements are then checked by a new match in each arm. By
repeating this process, we end up with a series of nested matches each
checking one element of the original scrutinee, which is equivalent to
the original match expression, and which lowers directly to SWITCH_EXPR
tree nodes (since these can only switch on one thing).

The resulting SWITCH_EXPRs may be messy, and unoptimal, but we can rely
on later optimizations in GENERIC and GIMPLE to clean them up for us.

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