New lower-level reading functions

[nhz2]

This PR exports three new functions useful for lower-level reading of raw entry data.

zip_compression_method(x::HasEntries, i::Integer)::UInt16

Return the compression method used for entry i.
See https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT for a current list of methods.
Only Store(0x0000) and Deflate(0x0008) supported for now.
Note: if the zip file was corrupted, this might be wrong.

zip_general_purpose_bit_flag(x::HasEntries, i::Integer)::UInt16

Return the general purpose bit flag for entry i.

See https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT
for a description of the bits.

zip_entry_data_offset(r::ZipReader, i::Integer)::Int64

Return the offset of the start of the compressed data for entry i from
the start of the buffer in r.
Throw an error if the local header is invalid.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.61%. Comparing base (30cf2f7) to head (49ce34a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #68   +/-   ##
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  Coverage   99.61%   99.61%           
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  Files           6        6           
  Lines         783      784    +1     
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+ Hits          780      781    +1     
  Misses          3        3           

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