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

idea/testData/checker/QualifiedExpressions.kt

@@ -1,13 +1,13 @@
 package qualified_expressions
 
-fun test(s: String?) {
-   val <warning>a</warning>: Int = <error>s?.length</error>
-   val b: Int? = s?.length
-   val <warning>c</warning>: Int = s?.length ?: -11
-   val <warning>d</warning>: Int = s?.length ?: <error>"empty"</error>
-   val e: String = <error>s?.length</error> ?: "empty"
-   val <warning>f</warning>: Int = s?.length ?: b ?: 1
+fun test(s: IntRange?) {
+   val <warning>a</warning>: Int = <error>s?.start</error>
+   val b: Int? = s?.start
+   val <warning>c</warning>: Int = s?.start ?: -11
+   val <warning>d</warning>: Int = s?.start ?: <error>"empty"</error>
+   val e: String = <error>s?.start</error> ?: "empty"
+   val <warning>f</warning>: Int = s?.end ?: b ?: 1
    val <warning>g</warning>: Boolean? = e.startsWith("s")//?.length
 }
 
-fun String.startsWith(<warning>s</warning>: String): Boolean = true
+fun String.startsWith(<warning>s</warning>: String): Boolean = true