feat(core): use class references as keys (container)

[kamilmysliwiec]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

The container uses class references as providers/controllers/injectables keys instead of their names.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 078b58e0-e3d1-42d2-bc07-d6e6557503d8

• 75 of 77 (97.4%) changed or added relevant lines in 11 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 94.731%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/core/injector/injector.ts	8	9	88.89%
packages/core/injector/module.ts	23	24	95.83%

Totals
Change from base Build c2030d95-6168-498b-8379-a80117eec7e0:	-0.01%
Covered Lines:	5070
Relevant Lines:	5352

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💛 - Coveralls