Allow compressed data in R data files

[timholy]

Fixes #225
Fixes #32
xref JuliaStats/RDatasets.jl#117

For those interested in the details: the 1.5.x-and-earlier version of FileIO checked hasmagic on the format, and if it was a recognized extension without "magic" then it blindly trusted the format. The rewrite does the same. However, it turns out that previously hasmagic returned false if the format had a detection function rather than a set of magic bytes, whereas in my "cleaned up" implementation I only return false if magic is literally empty. Hence the old version checked when the format was identifed via a list of bytes but not when it was detected by a registered detection function.

For the record, I think the old approach wasn't really correct: if you've registered a detection function, that should be the same as registering explicit bytes. But it's just another piece of havoc created by the rewrite which in retrospect clearly should have been FileIO 2. 😢

[codecov]

Codecov Report

Merging #308 (8049073) into master (3db3315) will decrease coverage by 0.19%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #308      +/-   ##
==========================================
- Coverage   89.59%   89.40%   -0.20%     
==========================================
  Files          10       10              
  Lines         596      623      +27     
==========================================
+ Hits          534      557      +23     
- Misses         62       66       +4     

Impacted Files	Coverage Δ
src/registry.jl	87.75% <96.55%> (+1.92%)	⬆️
src/deprecated.jl	52.17% <0.00%> (-6.53%)	⬇️

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