Additional cleanup with Go 1.13 as minimal version (google#295)

Add //go:build lines (google#285)

Starting with Go 1.17, //go:build lines are preferred over // +build
lines, see https://golang.org/doc/go1.17#build-lines and
https://golang.org/design/draft-gobuild for details.

This change was generated by running Go 1.17 go fmt ./... which
automatically adds //go:build lines based on the existing // +build
lines.

Also update the corresponding GitHub action to use Go 1.17 gofmt.

Print as text if mostly text (google#258)

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 google#257

Fix reporter verbosity bug (google#253)

FormatDiff should only set the verbosity to 3 if the current verbosity
is lower than 3. Otherwise, it may remove an intended higher verbosity
setting causing the reporter output to not differentiate between
two large values that are different at the end.

While we are at it, increase the maxVerbosityPreset to 6.

Impose verbosity limit when formatting map keys (google#248)

Map keys should have a sensible verbosity limit imposed,
otherwise the reporter can end up printing a massive data structure
that cannot reasonably fit in memory.

Fix Diff documentation (google#237)

The description inaccurately describes the operation of Diff,
which is y - x, where a '+' prefix denotes elements added from y
and a '-' prefix denotes elements removed from x.

For example:
	// Consider this call to Diff and its result.
	                  x  y
	cmp.Diff({b:2, c:3}, {a:1, b:2}) => {+a:1, b:2, -c:3}

	// Consider the same in mathematical notation.
	         y - x
	{a:1, b:2} - {b:2, c:3} = {+a:1, b:2, -c:3}

Suggest use of cmpopts.EquateErrors (google#234)

If cmp panics because it is trying to access an unexported field,
specially suggest the use of cmpopts.EquateErrors if the parent type
implements the error interface.

Fixes google#233

Use triple-quote formatting for multiline strings (google#229)

For strings, []bytes containing text data, Error method output, and
String method output, use the triple-quoted syntax.
This improves readability by presenting the data more naturally
compared to a single-line quoted string with many escaped characters.

Improve reporting of values with cycles (google#217)

Previously, the reporter could handle formatting values with cycles
in that it did not crash with a stack overflow. However, the output
was not particularly understandable as it did not surface to the user
why a particular value was truncated, and if it was truncated due
to a cyclic reference, what was the referent.

This change annotates the reporter tree with pointer information
so that a later pass can inject reference information if it is needed
to produce more understandable output.

Consider the following example:
  map[string]*cmp_test.CycleAlpha{
  	"Foo": &⟪ref#0⟫{
  		Name: "Foo",
  		Bravos: map[string]*cmp_test.CycleBravo{
  			"FooBravo": &{
- 				ID:     101,
+ 				ID:     0,
  				Name:   "FooBravo",
  				Mods:   100,
  				Alphas: {"Foo": &⟪ref#0⟫(...)},
  			},
  		},
  	},
  }

This graph contains a cycle. To ensure that a graph can be formatted,
the cycle is truncated as indicated with: &⟪ref#0⟫(...).
The referent was identified earlier with: &⟪ref#0⟫{...}.

Format units in decimal except bytes (google#199)

In general, decimal formatting is preferred except for []byte
where hexadecimal is preferred for individual elements.

Fixes google#185