Rewrite unsafeStringToBytes to satisfy 1.16 vet check

[robbertvanginkel]

Fixes #465.

According to https://golang.org/pkg/unsafe/#Pointer (6), one of the only safe ways to use pointers:

Conversion of a reflect.SliceHeader or reflect.StringHeader Data field to or from Pointer.
...
In general, reflect.SliceHeader and reflect.StringHeader should be used only as *reflect.SliceHeader and *reflect.StringHeader pointing at actual slices or strings, never as plain structs. A program should not declare or allocate variables of these struct types.

This rewrites the unsafe to only modify to not allocate a reflect.SliceHeader manually making the vet check happy.

[codecov]

Codecov Report

Merging #466 (2632889) into dev (b7d2a80) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #466      +/-   ##
==========================================
+ Coverage   78.86%   78.96%   +0.10%     
==========================================
  Files         125      125              
  Lines       16001    15981      -20     
==========================================
+ Hits        12619    12620       +1     
+ Misses       2077     2056      -21     
  Partials     1305     1305              

Impacted Files	Coverage Δ
wire/unsafe.go	100.00% <100.00%> (ø)
wire/type_string.go	0.00% <0.00%> (ø)
ast/basetypeid_string.go	75.00% <0.00%> (+51.92%)	⬆️

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[abhinav]

Thanks for fixing!