Optimizing "constant_string".Length

[jamesqo]

Lots of times for readability I use "constant_string".Length instead of 15 in my code. For example:

string TextAfterFoo(string s) =>
    s.Substring(s.IndexOf("Foo") + "Foo".Length); // a lot more readable than `3`

It would be nice if RyuJIT could optimize this as a JIT-time constant, so a string wouldn't have to actually be loaded and called Length on.

category:cq
theme:basic-cq
skill-level:intermediate
cost:medium

[EgorBo]

Implemented in #1378