Enable packing and unpacking structures with user-specified gaps between fields

[kagalenko-m-b]

Following the discussion in #34, I implemented the suggested syntax for packing to/unpacking from a binary buffer while skipping some parts of it. The usage may be illustrated by this fragment from runtests.jl:

@io struct RawData
    A::UInt32
    dummy_1::UInt8
    dummy_2::UInt16
    B::UInt16
    dummy_3::UInt32
    C::UInt128
    dummy_4::UInt16
    dummy_5::UInt64
    D::UInt8
end align_packed

@io struct GappedStruct
    A::UInt32
    _::3
    B::UInt16
    _::4
    C::UInt128
    _::2
    _::8
    _D::UInt8
end align_packed

A,B,C,D = 1,2,3,4
gapped_struct =  GappedStruct(A, B, C, D)
raw_data = RawData(A, 0xBE, 0xBEEF, B, 0xDEADBEEF, C,  0xBEEF, 0xDEADBEEFDEADBEEF,  D);
unpacked_raw_data = RawData(A, 0, 0, B, 0, C, 0, 0, D);

buf = IOBuffer()
pack(buf, raw_data)
seekstart(buf)
@test unpack(buf, GappedStruct) == gapped_struct
buf = IOBuffer(zeros(UInt8, packed_sizeof(GappedStruct)), read=true, write=true)
pack(buf, gapped_struct)
seekstart(buf)
@test unpack(buf, GappedStruct) == gapped_struct
seekstart(buf)
@test unpack(buf, RawData) == unpacked_raw_data

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 95.75%. Comparing base (1b79251) to head (4d73132). :warning: Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
+ Coverage   94.53%   95.75%   +1.22%     
==========================================
  Files           1        1              
  Lines         128      165      +37     
==========================================
+ Hits          121      158      +37     
  Misses          7        7              

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