Remove some more deprecated APIs.

These were already deprecated in 2.99 (or earlier).
Use a nested example group.

- Avoids `warning: method redefined; discarding old executed_examples_of`
- Produces better doc output
Merge pull request #1281 from rspec/remove-more-deprecations

Remove more deprecations
Tag slow specs.

In a new project, I would have split these "integration" tests out from
"unit" tests. That still may be worthwhile, but this is the first step
regardless.
Merge pull request #1285 from rspec/tag-slow-specs

Tag slow specs.
Remove sleeps from formatter specs

sleep not being deterministic has caused some test failures.
Fixes #1282
Merge pull request #1288 from rspec/remove-sleeping-in-specs

Remove sleeps from formatter specs
Refactor: remove special cases from configuration options.

It's easiest to leverage the same "order then set configs"
logic of `process_options_into` rather than treating
libs and requires as special.
Rename `parse_options` to `options` and have it return a memoized value.

The old way was odd. It created an ordering dependency
(you had to call `parse_options` after instantiating the
object but before doing anything else) and calling it a second
time had the side effect of re-parsing the args, which wasn't
needed or intended.

It's far easier/better to make `options` do the parsing
and memoize the result.
Allow `files_to_run` to be used in `spec_helper` when loaded early via `-r`.

This changes it to be lazily rather than eagerly computed.
This is important to allow `pattern` to be set from a `spec_helper`
file loaded via `-r`.
Merge pull request #1289 from rspec/fix-spec-helper-files-to-run

Fix for #1280
Remove obselete spec.txt file.
formatter notifications
send notification objects from reporter
upgrade deprecation formatter
upgrade legacy formatter
move notifications into own file
rework formatter specs to use actual notifications
refactor deprecation notification back to struct
used is a private method on seed