Make fzf pass go vet

Add String() methods to types, so they can be printed with %s. Change
some %s format specifiers to %v, when the default string representation
is good enough. In Go 1.10, `go test` triggers a parallel `go vet`. So
this also makes fzf pass `go test`.

Close #1236
Close #1219

src/util/chars.go

@@ -1,6 +1,7 @@
 package util
 
 import (
+	"fmt"
 	"unicode"
 	"unicode/utf8"
 	"unsafe"
@@ -94,6 +95,11 @@ func (chars *Chars) Length() int {
 	return len(chars.slice)
 }
 
+// String returns the string representation of a Chars object.
+func (chars *Chars) String() string {
+	return fmt.Sprintf("Chars{slice: []byte(%q), inBytes: %v, trimLengthKnown: %v, trimLength: %d, Index: %d}", chars.slice, chars.inBytes, chars.trimLengthKnown, chars.trimLength, chars.Index)
+}
+
 // TrimLength returns the length after trimming leading and trailing whitespaces
 func (chars *Chars) TrimLength() uint16 {
 	if chars.trimLengthKnown {