add missing group element invariant checks

The group element checks `secp256k1_{ge,gej}_verify` have first been
implemented and added in commit f202667
(PR bitcoin-core#1299). This commit adds additional verification calls in group
functions, to match the ones that were originally proposed in commit
09dbba5 of WIP-PR bitcoin-core#1032 (which is
obviously not rebased on bitcoin-core#1299 yet).

Also, for easier review, all functions handling group elements are
structured in the following wasy for easier review (idea suggested by
Tim Ruffing):

- on entry, verify all input ge, gej (and fe)
- empty line
- actual function body
- empty line
- on exit, verify all output ge, gej

Co-authored-by: Peter Dettman <peter.dettman@gmail.com>
Co-authored-by: Tim Ruffing <crypto@timruffing.de>