patch 9.0.0449: there is no easy way to translate a key code into a string

Problem:    There is no easy way to translate a string with a key code into a
            readable string.
Solution:   Add the keytrans() function. (closes #11114)

runtime/doc/builtin.txt

@@ -325,6 +325,8 @@ js_encode({expr})		String	encode JS style JSON
 json_decode({string})		any	decode JSON
 json_encode({expr})		String	encode JSON
 keys({dict})			List	keys in {dict}
+keytrans({string})		String	translate internal keycodes to a form
+					that can be used by |:map|
 len({expr})			Number	the length of {expr}
 libcall({lib}, {func}, {arg})	String	call {func} in library {lib} with {arg}
 libcallnr({lib}, {func}, {arg})	Number	idem, but return a Number
@@ -5205,6 +5207,16 @@ keys({dict})						*keys()*
 		Can also be used as a |method|: >
 			mydict->keys()
 
+keytrans({string})					*keytrans()*
+		Turn the internal byte representation of keys into a form that
+		can be used for |:map|.  E.g. >
+			:let xx = "\<C-Home>"
+			:echo keytrans(xx)
+<			<C-Home>
+
+		Can also be used as a |method|: >
+			"\<C-Home>"->keytrans()
+
 <							*len()* *E701*
 len({expr})	The result is a Number, which is the length of the argument.
 		When {expr} is a String or a Number the length in bytes is