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/libcmd/repl.cc

@@ -112,7 +112,7 @@ NixRepl::NixRepl(const SearchPath & searchPath, nix::ref<Store> store, ref<EvalS
     : AbstractNixRepl(state)
     , debugTraceIndex(0)
     , getValues(getValues)
-    , staticEnv(new StaticEnv(false, state->staticBaseEnv.get()))
+    , staticEnv(new StaticEnv(nullptr, state->staticBaseEnv.get()))
     , historyFile(getDataDir() + "/nix/repl-history")
 {
 }