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 *
These simply use StringBuilder::appendvf() internally which is not
optimal in terms of heap allocations, but simple enough and I don't
think they are performance sensitive functions anyway.
