cue: hoist literal string parsing into new package

This allows the parser and potentially other packages
to use this code without causing cycles.

Change-Id: I3df9f4a83109757f959adaecdccbc144a4975f61
diff --git a/pkg/strconv/manual.go b/pkg/strconv/manual.go
index e179ad4..cad0a30 100644
--- a/pkg/strconv/manual.go
+++ b/pkg/strconv/manual.go
@@ -14,11 +14,13 @@
 
 package strconv
 
+import "cuelang.org/go/cue/literal"
+
 // Unquote interprets s as a single-quoted, double-quoted,
 // or backquoted CUE string literal, returning the string value
 // that s quotes.
 func Unquote(s string) (string, error) {
-	return Unquote(s)
+	return literal.Unquote(s)
 }
 
 // TODO: replace parsing functions with parsing to apd