stdenv: Improve performance

| stat                   | before          | after           | Δ               | Δ%      |
|------------------------|-----------------|-----------------|-----------------|---------|
| cpuTime                | 513.67          | 507.77          | ↘ 5.90          | -1.15%  |
| envs-bytes             | 20,682,847,968  | 20,628,961,616  | ↘ 53,886,352    | -0.26%  |
| envs-elements          | 1,054,735,104   | 1,051,395,620   | ↘ 3,339,484     | -0.32%  |
| envs-number            | 765,310,446     | 763,612,291     | ↘ 1,698,155     | -0.22%  |
| gc-heapSize            | 53,439,602,688  | 51,711,545,344  | ↘ 1,728,057,344 | -3.23%  |
| gc-totalBytes          | 113,062,066,672 | 112,139,998,240 | ↘ 922,068,432   | -0.82%  |
| list-bytes             | 3,118,249,784   | 3,118,249,784   | 0               |         |
| list-concats           | 52,834,140      | 52,834,140      | 0               |         |
| list-elements          | 389,781,223     | 389,781,223     | 0               |         |
| nrAvoided              | 968,097,988     | 991,889,795     | ↗ 23,791,807    | 2.46%   |
| nrFunctionCalls        | 697,259,792     | 697,259,792     | 0               |         |
| nrLookups              | 510,257,062     | 338,275,331     | ↘ 171,981,731   | -33.70% |
| nrOpUpdateValuesCopied | 1,446,690,216   | 1,446,690,216   | 0               |         |
| nrOpUpdates            | 68,504,034      | 68,504,034      | 0               |         |
| nrPrimOpCalls          | 429,464,805     | 429,464,805     | 0               |         |
| nrThunks               | 1,009,240,391   | 982,109,100     | ↘ 27,131,291    | -2.69%  |
| sets-bytes             | 33,524,722,928  | 33,524,722,928  | 0               |         |
| sets-elements          | 1,938,309,212   | 1,938,309,212   | 0               |         |
| sets-number            | 156,985,971     | 156,985,971     | 0               |         |
| sizes-Attr             | 16              | 16              | 0               |         |
| sizes-Bindings         | 16              | 16              | 0               |         |
| sizes-Env              | 16              | 16              | 0               |         |
| sizes-Value            | 24              | 24              | 0               |         |
| symbols-bytes          | 2,151,298       | 2,151,298       | 0               |         |
| symbols-number         | 159,707         | 159,707         | 0               |         |
| values-bytes           | 30,218,194,248  | 29,567,043,264  | ↘ 651,150,984   | -2.15%  |
| values-number          | 1,259,091,427   | 1,231,960,136   | ↘ 27,131,291    | -2.15%  |

> Accessing the lexical scope directly should be more efficient, yes, because it changes from a binary search (many lookups) to just two memory accesses
> correction: one short linked list + one array access
> oh and you had to do the lexical scope lookup anyway for lib itself
> so it really does save a binary search at basically no extra cost

- roberth

after seeing the stats

> Oooh nice. I did not consider that more of the maybeThunk optimization becomes effective (nrAvoided). Those lookups also caused allocations!

- roberth

Left `lib.generators` and `lib.strings` alone because they're only used
once.

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