Left shifts of negative values is undefined in C. This happens a lot in
the Ed25519 code. Cast to unsigned first, then cast the result back to
signed where necessary.
These Windows include lines are capitalized, which causes the build to fail
when cross-compiling from Linux to Windows using MinGW as the MinGW headers
are entirely lower case.
This uses the portable Ed25519 library made by Orson Peters, which in turn uses
the reference implementation made by Daniel J. Bernstein.
This implementation also allows Ed25519 keys to be used for key exchange, so
there is no need to add a separate implementation of Curve25519.
