Meson migration

[caseif]

At some point I'd like to move away from CMake in favor of Meson as it has the potential to greatly simplify the project's build scripts. Unfortunately, there are currently blockers that this issue is meant to track.

---

configure_file can't output to a subdirectory (mesonbuild/meson#2320)

This presents a problem when configuring module_defs.hpp in the core module, as the configured file is expected to live under internal/core/ according to the project's include directory structure. There is a workaround in the relevant GH issue, but it seems really unwieldy and introduces completely unnecessary complexity (which is what I'm looking to reduce in switching to Meson).