winpr/crypt api changes and memory leak fixes

winpr/crypt api changes and memory leak fixes

- winpr_HMAC_New() now just returnes the opaque WINPR_HMAC_CTX* pointer
  which has to be passed to winpr_HMAC_Init() for (re)initialization
  and since winpr_HMAC_Final() no more frees the context you always have to
  use the new function winpr_HMAC_Free() once winpr_HMAC_New() has succeded

- winpr_Digest_New() now just returns the opaque WINPR_DIGEST_CTX* pointer
  which has to be passed to winpr_Digest_Init() for (re)initialization
  and since winpr_Digest_Final() no more frees the context you always have to
  use the new function winpr_Digest_Free() once winpr_Digest_New() has succeded