internal/conv: fix wrong string to bytes implementation

[cuonglm]

Description

UnsafeStrToBytes is currently not safe for -d=checkptr=2, since when it
cast from smaller struct (string) to bigger struct ([]byte). That causes
checkptr complains as the casting straddle multiple heap objects.

To fix this, we have to get the string header first, then use its fields
to construct the slice.

New implementation performs the same speed with the old (wrong) one.

name                old time/op    new time/op    delta
UnsafeStrToBytes-8    25.7ns ± 1%    25.7ns ± 3%   ~     (p=0.931 n=10+17)

name                old alloc/op   new alloc/op   delta
UnsafeStrToBytes-8     7.00B ± 0%     7.00B ± 0%   ~     (all equal)

name                old allocs/op  new allocs/op  delta
UnsafeStrToBytes-8      0.00           0.00        ~     (all equal)

While at it, also simplify UnsafeBytesToStr implementation, since when
we can pass the slice directly to unsafe.Pointer, instead of getting the
slice header first.

---

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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

[cuonglm]

cc @odeke-em

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@1bb71f8). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #9141   +/-   ##
=========================================
  Coverage          ?   58.80%           
=========================================
  Files             ?      583           
  Lines             ?    32748           
  Branches          ?        0           
=========================================
  Hits              ?    19257           
  Misses            ?    11214           
  Partials          ?     2277           

Impacted Files	Coverage Δ
internal/conv/string.go	100.00% <100.00%> (ø)

[odeke-em]

LGTM, thank you @cuonglm! Thank you @alessio for the review!