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

doc/manual/rl-next/env-size-reduction.md

@@ -0,0 +1,7 @@
+---
+synopsis: Reduce eval memory usage and wall time
+prs: 9658
+---
+
+Reduce the size of the `Env` struct used in the evaluator by a pointer, or 8 bytes on most modern machines.
+This reduces memory usage during eval by around 2% and wall time by around 3%.

doc/manual/rl-next/with-error-reporting.md

@@ -0,0 +1,31 @@
+---
+synopsis: Better error reporting for `with` expressions
+prs: 9658
+---
+
+`with` expressions using non-attrset values to resolve variables are now reported with proper positions.
+
+Previously an incorrect `with` expression would report no position at all, making it hard to determine where the error originated:
+
+```
+nix-repl> with 1; a  
+error:
+       … <borked>
+
+         at «none»:0: (source not available)
+
+       error: value is an integer while a set was expected
+```
+
+Now position information is preserved and reported as with most other errors:
+
+```
+nix-repl> with 1; a
+error:
+       … while evaluating the first subexpression of a with expression
+         at «string»:1:1:
+            1| with 1; a
+             | ^
+
+       error: value is an integer while a set was expected
+```