Implement OEB in terms of file.Buffer

[rjkroege]

Previous changes have added a newer alternative to file.File: file.Buffer that provides a more efficient backing storage mechanism for textual data with undo. This CL adds an adapter that implements ObservableEditableBuffer in terms of file.Buffer. Noteworthy changes:

• supporting the oeb callback logic for Undo/Reo from file.Buffer
• track the bounds of Undo/Redo action
• track undoable file name changes in file.Buffer
• relocate oeb callbacks where possible to do so
• modify oeb tests to invoke both the file.File and file.Buffer implementations

Provides the core of #97 -- a more idiomatic text buffer implementation.

[codecov[bot]]

Codecov Report

Merging #428 (257d989) into master (a787c7b) will increase coverage by 0.32%. The diff coverage is 78.04%.

@@            Coverage Diff             @@
##           master     #428      +/-   ##
==========================================
+ Coverage   56.64%   56.97%   +0.32%     
==========================================
  Files          56       56              
  Lines       10670    10781     +111     
==========================================
+ Hits         6044     6142      +98     
- Misses       4189     4199      +10     
- Partials      437      440       +3     

Impacted Files	Coverage Δ
text.go	49.12% <ø> (ø)
file/buffer_adapter.go	56.06% <54.68%> (+6.06%)	:arrow_up:
file/undo.go	88.59% <91.42%> (+3.44%)	:arrow_up:
file/observable_editable_buffer.go	75.25% <94.11%> (+3.82%)	:arrow_up:
file/file.go	86.00% <100.00%> (+1.43%)	:arrow_up:
file/rune_array.go	53.33% <0.00%> (+3.33%)	:arrow_up:

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