SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
This enables a nice warning in case a function becomes dead code. Also,
in the case of test-crypto.cpp, I took the liberty to add the prefix 'g_'
to the global event loop.