We've decided to adopt the ts_ prefix on all exported C functions in
order to avoid having symbol conflicts with future postgres functions.
We've already started using this prefix on new functions and this commit
adds the prefix to to the old functions.
The source tree under `test/` should mirror the directory structure
under `src/`. However, the loader mock was currently in
`test/loader-mock/src` instead of `test/src/loader`, which has now
been fixed. The loader mock was also built from CMake files in
`src/loader` since that made it inherit a number of required build
definitions used by the non-mocked loader. To avoid having to redefine
these definitions for the build in `test/`, they have been split out
into a `build-defs.cmake` file that can be included in both the
regular loader and the mock build.
The build system has also been cleaned up in other ways. For instance,
header files are no longer added as dependencies unless they are,
e.g., generated headers. Test-specific code is built separately from
the main extension and only linked in case of debug builds.
