s/seperate/separate/

Even with an optimal diffing algoritm, coalescing adjacent edit groups
may cause the corresponding pair of strings for an edit group to have
leading or trailing spans of equal elements.

While this is technically a correct representation of a diff,
it is a suboptimal outcome. As such, scan through all unequal groups and
move leading/trailing equal spans to the preceding/succeeding equal group.

Before this change:
    strings.Join({
      "org-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=aa",
  -   ",#=_value _value=2 ",
  +   " _value=2 ",
      `11 org-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=bb`,
  -   ",#=_value _value=2 2",
  +   " _value=2 2",
      `1  org-4747474747474747,bucket-4242424242424242:m,tag1=b,tag2=cc`,
  -   ",#=_value",
      ` _value=1 21 org-4747474747474747,bucket-4242424242424242:m,tag1`,
      "=a,tag2",
  -   "=dd,#=_value",
  +   "=dd",
      ` _value=3 31 org-4747474747474747,bucket-4242424242424242:m,tag1`,
  -   "=c,#=_value",
  +   "=c",
      ` _value=4 41 `,
    }, "")

After this change:
    strings.Join({
      "org-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=aa",
  -   ",#=_value",
      ` _value=2 11 org-4747474747474747,bucket-4242424242424242:m,tag1`,
      "=a,tag2=bb",
  -   ",#=_value",
      ` _value=2 21 org-4747474747474747,bucket-4242424242424242:m,tag1`,
      "=b,tag2=cc",
  -   ",#=_value",
      ` _value=1 21 org-4747474747474747,bucket-4242424242424242:m,tag1`,
      "=a,tag2=dd",
  -   ",#=_value",
      ` _value=3 31 org-4747474747474747,bucket-4242424242424242:m,tag1`,
      "=c",
  -   ",#=_value",
      ` _value=4 41 `,
    }, "")

Avoid diffing by lines if it turns out to be significantly less
efficient than diffing by bytes.

Before this change:
    (
    	"""
  - 	d5c14bdf6bac81c27afc5429500ed750
  - 	25483503b557c606dad4f144d27ae10b
  - 	90bdbcdbb6ea7156068e3dcfb7459244
  - 	978f480a6e3cced51e297fbff9a506b7
  + 	Xd5c14bdf6bac81c27afc5429500ed750
  + 	X25483503b557c606dad4f144d27ae10b
  + 	X90bdbcdbb6ea7156068e3dcfb7459244
  + 	X978f480a6e3cced51e297fbff9a506b7
    	"""
    )

After this change:
    strings.Join({
  + 	"X",
    	"d5c14bdf6bac81c27afc5429500ed750\n",
  + 	"X",
    	"25483503b557c606dad4f144d27ae10b\n",
  + 	"X",
    	"90bdbcdbb6ea7156068e3dcfb7459244\n",
  + 	"X",
    	"978f480a6e3cced51e297fbff9a506b7\n",
    }, "")

The previous heuristic of treating strings as binary data
if it contains any invalid UTF-8 was too strict.
Loosen the heuristic to check if most of the characters
are printable text.

Fixes #257