Update readbytes! to avoid using internal API of IOBuffer less

[vtjnash]

Note that this claims to be supported for all GenericIOBuffer, but Base only defines this function on a normal IOBuffer for which pointer(data) is contiguous space

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 82.62%. Comparing base (040cf0a) to head (6f3353f).

Files	Patch %	Lines
src/Streams.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1148      +/-   ##
==========================================
- Coverage   82.67%   82.62%   -0.05%     
==========================================
  Files          32       32              
  Lines        3054     3051       -3     
==========================================
- Hits         2525     2521       -4     
- Misses        529      530       +1     

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[vtjnash]

Alright, I think I fixed the remaining test issues so this is passing on all versions now

[oscardssmith]

bumping this.