internal: create package for unsafe bytes convertion

[robert-zaremba]

Description

We start to duplicate a code for unsafe and with 0 allocation bytes <-> string convertion. This PR moves the code to a single place - an internal conv package.

Ref: #8719 (comment)

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• 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
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #8733 (5ae6bae) into master (b1c72fd) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #8733      +/-   ##
==========================================
- Coverage   61.40%   61.39%   -0.01%     
==========================================
  Files         671      672       +1     
  Lines       38348    38346       -2     
==========================================
- Hits        23546    23544       -2     
+ Misses      12335    12334       -1     
- Partials     2467     2468       +1     

Impacted Files	Coverage Δ
x/auth/legacy/legacytx/stdsign.go	82.97% <ø> (ø)
store/cachekv/store.go	88.17% <50.00%> (+1.04%)	⬆️
x/auth/legacy/legacytx/stdtx.go	80.85% <50.00%> (-1.38%)	⬇️
internal/conv/string.go	100.00% <100.00%> (ø)
types/address/hash.go	100.00% <100.00%> (ø)

[alessio]

ACK'd

[tac0turtle]

🔥