Support non-UTF-8 paths in FileSystem

[swankjesse]

We’ve designed Path to support this, but we don’t have tests that exercise it, and I don’t have confidence it’ll actually work.

In particular:

• Native: Kotlin’s built-in C bindings do implicit UTF-8 encoding and decoding. We need alternate APIs to access other encodings.
• JVM: the JVM’s java.io and java.nio.file APIs use Java Strings. Presumably these are broken on exotic file paths?
• JS: the Node API we’re using uses JavaScript Strings. Presumably these are also broken by exotic file paths.