Various improvement in the serialization code

Summary:

• Use fixed size types or size_t
• Use C++11 style for loops
• Use braces consistently

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1286