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
multiprecision_int is a wrapper around boost::multiprecision::cpp_int that adds
no functionality. The intent is to allow forward declration; cpp_int is so
complicated that just finding out what its true type is a difficult exercise, as
it depends on many internal declarations.
Because cpp_int uses expression templates, the implementation has to explicitly
cast to the desired type in many places, otherwise the C++ compile is presented
with too many choices, especially in conjunction with data_value (which can
convert from many different types too).
