fix: Make lstripChars and rStripChars work with multibyte characters (google#663)

We're currently working with the length of the string itself, which is
the number of bytes. That's wrong in these functions which operate at
the level of characters. When applied to a string with a multibyte
character they can return the wrong result. For example, evaluating
`std.rstripChars('• foo\n', '\n')` results in `"• f"`.

What we need to do instead is get the number of runes, and remove one
rune at a time.

Author: iainlane

builtins.go

@@ -517,7 +517,8 @@ func builtinLstripChars(i *interpreter, str, chars value) (value, error) {
 				return nil, err
 			}
 			if member {
-				s := strType.getGoString()[1:]
+				runes := strType.getRunes()
+				s := string(runes[1:])
 				return builtinLstripChars(i, makeValueString(s), chars)
 			} else {
 				return str, nil
@@ -542,7 +543,8 @@ func builtinRstripChars(i *interpreter, str, chars value) (value, error) {
 				return nil, err
 			}
 			if member {
-				s := strType.getGoString()[:strType.length()-1]
+				runes := strType.getRunes()
+				s := string(runes[:len(runes)-1])
 				return builtinRstripChars(i, makeValueString(s), chars)
 			} else {
 				return str, nil

testdata/std.lstripChars.multibyte.golden

@@ -0,0 +1 @@
+"foo"

testdata/std.lstripChars.multibyte.jsonnet

@@ -0,0 +1 @@
+std.lstripChars('• foo', '• ')

testdata/std.lstripChars.multibyte.linter.golden

@@ -0,0 +1 @@
+"• foo"

testdata/std.rstripChars.multibyte.golden

@@ -0,0 +1 @@
+std.rstripChars('• foo\n', '\n')