read_header_int: allow leading spaces

[StefanKarpinski]

There are a number of changes in this PR, but the main one is the fix for #111 which is to allow leading spaces in integer fields since some old tar programs emitted these fields that way (contrary to the spec). The other substantial change is improved checking and reporting of errors in headers. Here is the list of all changes:

• read_header_int: allow leading spaces (fix #111)
• rename header_view => data
• read_standard_header: improve error checks & messages
• read_header_size: avoid modifying buffer & try/catch

[codecov[bot]]

Codecov Report

Merging #116 (45197a8) into master (343e923) will increase coverage by 0.79%. The diff coverage is 98.73%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
+ Coverage   96.05%   96.84%   +0.79%     
==========================================
  Files           4        4              
  Lines         684      729      +45     
==========================================
+ Hits          657      706      +49     
+ Misses         27       23       -4     

Impacted Files	Coverage Δ
src/extract.jl	97.97% <98.66%> (+1.39%)	:arrow_up:
src/create.jl	97.19% <100.00%> (ø)

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