Font subsetting and font optimization improvements

[gunnsth]

This PR is focused on improving some optimization methods, mostly for font optimization, but also some other approaches.

• Add NewFromContents to extractor to enable extracting from generic pdf contents / resources.
• IdentityEncoder - add rune tracking for subsetting
• Image compression: Try DCT and flate combined
• Content stream optimization: Remove unnecessary operands
• Font program reduction: Remove redundant tables from truetype fonts
• Font program subsetting: Remove redundant glyphs and reduce truetype fonts

Note that some golint fixes mixed in also.

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This change is 

[codecov[bot]]

Codecov Report

Merging #362 into development will decrease coverage by 6.23%. The diff coverage is 18.20%.

@@               Coverage Diff               @@
##           development     #362      +/-   ##
===============================================
- Coverage        62.39%   56.16%   -6.24%     
===============================================
  Files              236      239       +3     
  Lines            45802    46206     +404     
===============================================
- Hits             28580    25951    -2629     
- Misses           16549    16905     +356     
- Partials           673     3350    +2677     

Impacted Files	Coverage Δ
model/optimize/clean_contentstream.go	0.00% <0.00%> (ø)
model/optimize/clean_fonts.go	0.00% <0.00%> (ø)
model/optimize/optimizer.go	87.14% <0.00%> (-5.29%)	:arrow_down:
model/optimize/utils.go	0.00% <0.00%> (ø)
extractor/text.go	63.47% <25.00%> (-8.26%)	:arrow_down:
model/font_composite.go	58.44% <25.00%> (-11.93%)	:arrow_down:
internal/textencoding/identity.go	40.90% <50.00%> (-2.43%)	:arrow_down:
model/font.go	57.56% <50.00%> (-7.33%)	:arrow_down:
model/annotations.go	22.33% <54.54%> (-1.85%)	:arrow_down:
model/page.go	53.20% <82.35%> (-7.38%)	:arrow_down:
... and 164 more

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