Place public ECDSA API next to the internal methods

Place public ECDSA API next to the internal methods

It is hard to remember that ECDSA_do_{sign,verify}() call ecdsa_sign_sig().
Especially since the distinction to ECDSA_{sign,verify}() isn't clear from
the names. To add to the confusion, the public API is ordered differently
than the methods they call. So in this case it seems tidier to place the
public API next to the methods.

ok jsing