reduce the size of Env by one pointer

since `up` and `values` are both pointer-aligned the type field will
also be pointer-aligned, wasting 48 bits of space on most machines. we
can get away with removing the type field altogether by encoding some
information into the `with` expr that created the env to begin with,
reducing the GC load for the absolutely massive amount of single-entry
envs we create for lambdas. this reduces memory usage of system eval by
quite a bit (reducing heap size of our system eval from 8.4GB to 8.23GB)
and gives similar savings in eval time.

running `nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'`

before:

  Time (mean ± σ):      5.576 s ±  0.003 s    [User: 5.197 s, System: 0.378 s]
  Range (min … max):    5.572 s …  5.581 s    10 runs

after:

  Time (mean ± σ):      5.408 s ±  0.002 s    [User: 5.019 s, System: 0.388 s]
  Range (min … max):    5.405 s …  5.411 s    10 runs

src/libexpr/eval-inline.hh

@@ -73,8 +73,6 @@ Env & EvalState::allocEnv(size_t size)
 #endif
         env = (Env *) allocBytes(sizeof(Env) + size * sizeof(Value *));
 
-    env->type = Env::Plain;
-
     /* We assume that env->values has been cleared by the allocator; maybeThunk() and lookupVar fromWith expect this. */
 
     return *env;