Sprinkle constexpr where needed to make the default constructor,
move constructor, and destructor constexpr.
Add a test to verify.
This is needed to make a thread_local variable containing an
empty managed_bytes constinit, reducing thread-local guards.
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
We will need them to replace bytes with managed_bytes in some places in an
upcoming patch.
The change to configure.py is necessary because opearator<< links to to_hex
in bytes.cc.
