convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

Update CakeML code generation to accomodate CakeML changes

Removes Holmake cruft that doesn't make sense now that CakeML
doesn't construct intermediate heaps for subdirectories anymore.

CMake: Construct CakeML build files from templates

The behavior of DeclareCakeMLLib is unchanged, but it should be easier
to maintain the build files without them being inlined in a CMake file.