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 *
Until we get the goodness that C++ modules are supposed to be, let's try
to shave off some parse time using precompiled headers.
This commit only adds some very common AK headers, only to binaries,
libraries and the kernel (tests are not covered due to incompatibility
with AK/TestSuite.h).
This option is on by default, but can be disabled by passing
`-DPRECOMPILE_COMMON_HEADERS=OFF` to cmake, which will disable all
header precompilations.
This makes the build about 30 seconds faster on my machine (about 7%).
