Make CTxIn and CTxOut constructors defined inline

Summary: They aresimple ctors that pass data back and forth, so no point in preventing the compiler to optimize them away.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1406