Move codec.RegisterCrypto and codec.Cdc to new packages

[aaronc]

Description

In order to register the multisig type that @marbar3778 added in #6241 with amino (which is necessary for completing #6216), we need to move the global amino Cdc instance outside of the codec package to avoid cyclic dependencies.

This PR fixes this by:

• moving codec.RegisterCrypto to crypto/codec and having it reference the amino registration in crypto/types/multisig which was added in multisig: add type and bitarray #6241
• moving codec.Cdc to a new codec/legacy_global package (which has an intentionally ugly name) and deprecating it

---

For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[codecov]

Codecov Report

Merging #6330 into master will decrease coverage by 0.03%.
The diff coverage is 80.95%.

@@            Coverage Diff             @@
##           master    #6330      +/-   ##
==========================================
- Coverage   55.66%   55.63%   -0.04%     
==========================================
  Files         448      448              
  Lines       26956    26951       -5     
==========================================
- Hits        15005    14994      -11     
- Misses      10877    10883       +6     
  Partials     1074     1074              

[alexanderbez]

LGTM