mem: add Buffer.Slice()

[ash2k]

I have a CodecV2 that gives me a mem.BufferSlice (i.e. exposes what it gets from gRPC directly). I need to get another mem.BufferSlice from it that is a subslice. In some scenarios I need to slice the first and/or the last buffer to get only the subset of data. Hence, having a Slice() method on the Buffer type would be really useful.

Another use case: I want to use BufferSlice.Reader and implement a Peek()(or similar) to get a BufferSlice of the next N bytes. I don't want a []byte or [][]byte, I'd like to return this BufferSlice from a codecv2 and let gRPC free the buffers when it no longer needs them (i.e. I don't want to wrap those individual []byte into BufferSlices).

RELEASE NOTES:

• Add mem.Buffer.Slice() - new method to slice the buffer like a slice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.10%. Comparing base (e5563c6) to head (a405abf).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8977      +/-   ##
==========================================
+ Coverage   82.93%   83.10%   +0.16%     
==========================================
  Files         411      411              
  Lines       32704    32719      +15     
==========================================
+ Hits        27124    27192      +68     
+ Misses       4179     4146      -33     
+ Partials     1401     1381      -20     

Files with missing lines	Coverage Δ
mem/buffer_slice.go	96.45% <100.00%> (ø)
mem/buffers.go	89.81% <100.00%> (+1.64%)	⬆️

... and 32 files with indirect coverage changes

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