Windows 64-bit compilation (including type changes)

- C API: replace `unsigned long long` by `GtUint64`
- C API: replace `uint64_t` by `GtUint64`
- C API: replace `unsigned long` by `GtUword`
- C API: replace `long` by `GtWord`
- C API: replace `uint8_t` by `unsigned char`
- C API: replace `long double` by `double`

`uint64_t` and `uint8_t` were replaced in API headers only, the rest everywhere.
The relevant definitions for types and formatting chars are given in `core/types_api.h`.
I extended the `src_check` to check for the new rules.

Some rough edges remain, but `scripts/build_windows_64.sh` compiles successfully! (Closes #63)