Do not import the whole std namespace in script.cpp

Summary: It avoid namespace clashes. We can import symbol specifically if wanted, but not the whole namespace as it is prone to symbol clashes.

Test Plan:

make check

Reviewers: freetrader, sickpig, #bitcoin_abc, kyuupichan

Reviewed By: #bitcoin_abc, kyuupichan

Differential Revision: https://reviews.bitcoinabc.org/D145