Fixes #198.
Builds on top of #293.
Mechanical application of
for A in include/*.h src/*.h src/*.c src/modules/*/*.h; do sed -e 's/secp256k1_\([a-z_]*\)_t\([^a-zA-Z]\)/secp256k1_\1\2/g' <$A >/tmp/bla; mv /tmp/bla $A; sed -e 's/callback_t/secp256k1_callback/g' <$A >/tmp/bla; mv /tmp/bla $A; done