Point::steg_encode was leaving the 24 high bits of the buffer as zero.
It also ignored the size parameter.  The size parameter has now been removed,
the zeros fixed and a test added to make sure that it is fixed.

Per https://github.com/MystenLabs/ed25519-unsafe-libs, deprecate eddsa signing
with separate pubkey and privkey input.  Instead decaf_ed*_keypair_sign

This reverts commit 540d9b8158.

The commit appears to break the build

Apply Modern CMake best practices and use target_include_directories
rather than include_directories for defining where to find header files.
Where possible, scope was reduced to PRIVATE, indicating that the headers
are required only for compilation, not when linking against the target.

Additionally, the generated headers that are required for the decaf libs
to work have been added to their build interface. This means that other
projects that use CMake to build this project will correctly link against
the decaf and decaf-static targets.

The existing code checked if a directory existed and if it did, it would
not attempt to create it. However, this proved to be flaky on Mac and Linux.
The check would frequently report the directory as not existing, but the
attempt to create the directory would cause an exception.

- decaf_tables.c are back in the source tree to help cross-compilation