Boot the Builder pattern from GenesisBlock

[garious]

Problem

Rust's Builder pattern is an anti-pattern. We ran this experiment a while back with the MessageBuilder struct. It led to confusing code and redundant wrappers. It's especially ugly when you start using it in imperative code (let builder = builder.do_something()). In the case of GenesisBlock, it went the same way.

Summary of Changes

• Remove the Builder wrapper and add &mut self methods to GenesisBlock that move their input parameters instead of cloning them. Use the T { field*, ..T::default() } pattern.
• Change some function names from ambiguous names like genesis() to the form <verb>_<subject>.
• Purge term "primordial", because it's used the same was a "genesis" everywhere else.
• Removed some unused crate dependencies.

[codecov]

Codecov Report

Merging #6364 into master will decrease coverage by 1%.
The diff coverage is 77.6%.

@@           Coverage Diff            @@
##           master   #6364     +/-   ##
========================================
- Coverage    75.8%   74.7%   -1.1%     
========================================
  Files         219     219             
  Lines       42965   43495    +530     
========================================
- Hits        32568   32499     -69     
- Misses      10397   10996    +599