Remove unnecessary file
Change src-no-amb to pass around the env

eval and apply, rather than returning what they used to return, now return a
vector of their old return value and the environment after the operation.
Environments are immutable.

Problem: There's no way to implement amb in this system :(
Add src2 folder, which is the base pset4-amb code
Change src2 succeed continuations to take the env

succeed now takes val, env, fail instead of just val, fail. Environments are
now immutable and passed around to success continuations, rather than having a
global environment that's modified.

I'm not convinced that everything is 100% correct, but this is at least really
close to correct. It also passes tests/testset.icm.

src-no-amb is now obsolete.