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 *
Problem:
- `constexpr` functions are additionally decorated with `inline`
keyword. This is redundant since `constexpr` implies `inline`.
Solution:
- Remove redundancies.