Make CharSequence.length a function instead of property

And String.length as well.

This is done for JVM interoperability: java.lang.CharSequence is an open class
and has a function 'length()' which should be implemented in subclasses
somehow.

A minor unexpected effect of this is that String.length() is now a compile-time
constant (it wasn't such as a property because properties are not supported in
compile-time constant evaluation)

 #KT-3571 Fixed

js/js.libraries/src/core/string.kt

@@ -38,9 +38,6 @@ native public fun String.trim() : String = noImpl
 native("length")
 public val CharSequence.size: Int get() = noImpl
 
-library
-public fun CharSequence.length(): Int = noImpl
-
 library
 public fun CharSequence.isEmpty(): Boolean = noImpl