Closes ticket 27359.

This representation is meant to save memory in microdescriptors --
we can't use it in routerinfo_t yet, since those families need to be
encoded losslessly for directory voting to work.

This representation saves memory in three ways:
   1. It uses only one allocation per family.  (The old way used a
      smartlist (2 allocs) plus one strdup per entry.)
   2. It stores identity digests in binary, not hex.
   3. It keeps families in a canonical format, memoizes, and
      reference-counts them.

Part of #27359.

"ours" to avoid version bump.

"ours" to avoid version bump.

"ours" merge to avoid taking 25573 and 27686 again.

"ours" merge to avoid duplicating conflict resolution in merge of 24104

Resolved conflicts with the 26269 fix in 015fcd0e.

"ours" merge to avoid the 0.3.3 version of #27550.

Documentation for 28399, 28454, and previous fixes.

The mingw zstd from MSYS2 doesn't come with a pkg-config file.

Fixes 28454 on Tor 0.3.4.1-alpha.

Fixes 28399 on 0.3.4.1-alpha.

Stop installing MSYS2 packages.
We're compiling for mingw, so we only need mingw packages.

Run pacman in verbose mode.

Explicitly install pkg-config and xz, to future-proof our builds.

Diagnostics for 28399.