Source location for lambdas.
More missing numerics, spectral-norm runs.
Some shootout benchmarks, almost exactly as in Racket.
Add more benchmarks.
More prims.
Add simple version of benchmark.
More jit testing.
Make Module not an AST

It's currently not a meaningful AST anyway, as it does not have an
interpret method. Had to disable load to make it work, but load crashes
anyway when executing it atm.
store the surrounding lambda on most AST elements

useful for debugging and for the soon-to-come call graph reconstruction
do some more environment pruning

if the body of a let does not need a prefix of the outer environment,
remove that prefix before creating the environment that the let
produces.

could be extended if some parts of the rhss don't need the full
environment either.