Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
The copy constructor of small vector has a noexcept specifier, however
it calls `reserve(size_t)`, which can throw `std::bad_alloc`. This
causes issues when using it inside tests that use
alloc_failure_injector, but potentially could also float up in the
production.
Closes#9338
small_vector is a variation of std::vector<> that reserves a configurable
amount of storage internally, without the need for memory allocation.
This can bring measurable gains if the expected number of elements is
small. The drawback is that moving such small_vector is more expensive
and invalidates iterators as well as references which disqualifies it in
some cases.
