Draft: Accepting already open and limited IO types

[JonasIsensee]

This adds a method jldopen(::IO, mode; kwargs...)
that should take in already opened IOs and figure out what to do with them.

Currently, this PR implements a ReadOnlyBuffer that can wrap another IO object which only supports reading but not seeking. Therefore it reads linearly and buffers everything in between.

• figure out what to implement for reading
• figure out what to implement for writing
• How to handle a capable external stream (light-weight wrapper)
• How to figure out what an IO object can do (how to wrap it or error)
• Do something creative with the filename field in JLDFile

Referencing
#346
#233
#304

[codecov]

Codecov Report

Attention: Patch coverage is 85.26786% with 33 lines in your changes are missing coverage. Please review.

Project coverage is 86.51%. Comparing base (15eaa50) to head (440b29a).
Report is 28 commits behind head on master.

Files	Patch %	Lines
src/compression.jl	4.54%	21 Missing ⚠️
src/general_io.jl	94.27%	11 Missing ⚠️
src/JLD2.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #535      +/-   ##
==========================================
- Coverage   86.81%   86.51%   -0.30%     
==========================================
  Files          31       32       +1     
  Lines        4299     4539     +240     
==========================================
+ Hits         3732     3927     +195     
- Misses        567      612      +45     

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