Minor cosmetic fixes to changelog

[ci-skip]
Improve changelog.

- "Full Changelog", not "full changelog".
- Remove duplicate `is_expected` changelog entry.
  It's already listed in 2.99.0.beta2 enhancements.
- Revise `describe`/`context` entry. When it was written,
  the pending changes hadn't been implemented yet.
Add an option to use spec location if there is no call site.
documentation for RSpec::Core::World
pass along opts from warn_deprecation
Add missing space to message.
updates per discussion on pull request
Cleanup after #1337

Just dropping some lines off for those settings that are self
explanatory accessors.

[skip ci]
Don't include any test files in the gem.

I don't think anyone actually relies on that
feature and it makes the published size of the
gem more than twice as big (192 KB vs 90 KB).
Benchmark require_relative.

It appears not to make a difference.
But wait...using this old benchmark it does make a difference?
Benchmark the perf difference with a load path of different sizes.
Use new rspec-support requires.
Move `set` require to file where it is used.
One line per require file.
Updated travis build scripts (from rspec-dev)
Remove benchmark file.

This benchmark suggested that `require_relative` isn't
any faster than `require`, because it put `rspec/core/lib`
onto the load path as the first dir, which made `require`
for rspec-core files O(1) like `require_relative`.

In practice, people's load path will have many more directories
and this benchmark doesn't align with that reality.

[ci skip]
Fix generated spec_helper so we don't get double formatters.

The spec_helper generated by `rspec --init` could
cause both the progress and documentation formatter
to be used for a command like:

rspec path/to/spec.rb --format progress

This happened because formatters are loaded _after_
`--require` files, so the `config.formatters.none?`
check returned true even though a formatter had
been explicitly set at the command line. It's important
that requires come first so that custom formatters can
be appropriately loaded.
Add changelog entries.

[ci skip]