refactor(types/parser): parse by fragment and process in pipeline

[xcoulon]

Fragments are blocks of contiguous lines separated by blank lines.
Blank lines within delimited blocks are preserved (hence they do not
split the fragments)
Once a fragment is read, it is sent on a channel to be processed
by the next stages of the pipeline until the aggregator which combines
all the fragments into the resulting types.Document, which
can then be rendered

BREAKING CHANGES:

• using pointers on all structs in pkg/types
• removing types.Document.Attributes, holding attributes in context
  as parsing/rendering progresses.
• types.Paragraph.Lines ([][]interface{}) is replaced by
  types.Paragraph.Elements ([]interface{})
• all delimited blocks types are merged into types.DelimitedBlock
• all lists are merged into types.List
• removed the Level field in types.ListElement struct
• refactor document structure with header (level 0) and
  sections (level 1 to 5)

Fixes #843

Signed-off-by: Xavier Coulon [email protected]

[codecov]

Codecov Report

Merging #855 (8037812) into master (82bfd34) will increase coverage by 1.38%.
The diff coverage is 85.09%.

@@            Coverage Diff             @@
##           master     #855      +/-   ##
==========================================
+ Coverage   78.78%   80.16%   +1.38%     
==========================================
  Files          88       85       -3     
  Lines        5289     6499    +1210     
==========================================
+ Hits         4167     5210    +1043     
- Misses        694      806     +112     
- Partials      428      483      +55     

[xcoulon]

FWIW, running the benchs in libasciidoc_bench_test.go compared to the code tagged v0.6.0 gives the following improvements (via benchstat):

name                   old time/op    new time/op    delta
ParseBasicDocument-12    6.69ms ± 3%    0.68ms ± 3%  -89.79%  (p=0.000 n=9+9)
ParseLongDocument-12      295ms ± 2%      25ms ± 2%  -91.54%  (p=0.000 n=9+10)

name                   old alloc/op   new alloc/op   delta
ParseBasicDocument-12    2.01MB ± 0%    0.24MB ± 0%  -87.83%  (p=0.000 n=10+9)
ParseLongDocument-12     99.5MB ± 0%    11.5MB ± 0%  -88.44%  (p=0.000 n=9+10)

name                   old allocs/op  new allocs/op  delta
ParseBasicDocument-12     41.2k ± 0%      4.1k ± 0%  -89.97%  (p=0.001 n=8+9)
ParseLongDocument-12      2.06M ± 0%     0.20M ± 0%  -90.08%  (p=0.000 n=9+10)