Remove unneded simplecov formatter setup
Adjust line coverage expectation
Remove redundant rspec configuration
Add mutation covering specs for Runner::Collector
Fix Expression::Method#match_length
Set Thread.abort_on_exception to true
Add specification for Subject::Method#match_expressions
Fix pending specs for exceptional isolation behavior
Add spec to verify isolation works for crashing killforks
Silence warnings when intentionally cause them
Adjust specs for changed line numbers
Fix code style
Remove parallel use for isolation

* Parallel is not thread save, causing all sorts of issues when nesting.
Whitelist nested iterator use
Whitelist method with many statements
Fix the amount of parallel processes in use in CI environments

This should fix cases where mutants corpus tests runs into resource
problems.
Prepare v0.6.4

[fixes #265][fixes #261]
Add Subject#source_lines
WIP