Increase memory for 'All * Tests' configurations

Use the same parameters as for All Tests
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
Rename CharSequence.get to charAt

This is done for JVM interoperability. There's still a member function
String.get() and an extension function "get" on CharSequence

 #KT-1730 Fixed
 #KT-5389 Fixed
Add CharSequence.subSequence

This is done primarily for JVM interoperability, otherwise it's impossible to
inherit from CharSequence there. On JS subSequence at the moment just invokes
substring.

 #KT-5956 Fixed