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.

• [x] figure out what to implement for reading
• [x] 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

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[codecov[bot]]

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 :warning:
src/general_io.jl	94.27%	11 Missing :warning:
src/JLD2.jl	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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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