Extensive interface and operations tests for secp256k1_ec_pubkey_parse.
Extensive interface and operations tests for secp256k1_ec_pubkey_parse.

This also makes use of optional valgrind instrumentation if -DVALGRIND
 is set.

This also moves secp256k1.c above secp256k1.h in tests.c or otherwise
 we get non-null macros on the public functions which may defeat some
 of the VERIFY checks.

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