Refactor ParseScript logic to remove else conditions

Summary:
This makes the code easier to understand by unclumping the logic of the parsing and using C++11 style loops.

Depends on D1290

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1292