Modernize compact block code

Summary:

• indexes -> indices
• Use C++11 loops when possible
• Add braces
• Use C++ constructors instead C style casts.

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1134