Experiment with specialized lists

The idea is to have a compiler-specific list data type that
optimizes for small lists. Where the overhead of normal lists
is one Cons node per element, the overhead of `Lst` is
0 for lists of length <= 1, and 1 array node for longer lists.
Furthermore, we'll try to inline heavily, which is easier for
specialized data structures.

The main downsides are inefficient `tail` and `::`, in particular
for longer lists. Compiler code will need to be adapted to avoid these where
it is performance critical.

This also serves as test case for the inline related fixes in the
previous commits.

Author: odersky