Release 2.16: Faster Downloads and Lazy `replaceStrings`

doc/manual/source/release-notes/rl-2.16.md

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# Release 2.16 (2023-05-31)

* Speed-up of downloads from binary caches.
  The number of parallel downloads (also known as substitutions) has been separated from the [`--max-jobs` setting](../command-ref/conf-file.md#conf-max-jobs).
  The new setting is called [`max-substitution-jobs`](../command-ref/conf-file.md#conf-max-substitution-jobs).
  The number of parallel downloads is now set to 16 by default (previously, the default was 1 due to the coupling to build jobs).

* The function [`builtins.replaceStrings`](@docroot@/language/builtins.md#builtins-replaceStrings) is now lazy in the value of its second argument `to`. That is, `to` is only evaluated when its corresponding pattern in `from` is matched in the string `s`.

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Title: Release 2.16: Faster Downloads and Lazy `replaceStrings`

Summary

Release 2.16, dated 2023-05-31, introduces significant improvements including a speed-up for downloads from binary caches. This is achieved by separating the number of parallel downloads (`max-substitution-jobs`) from the `--max-jobs` setting and increasing the default parallel download count from 1 to 16. Additionally, the `builtins.replaceStrings` function is now lazy, evaluating its second argument `to` only when a corresponding pattern is matched in the input string `s`.