feature-complete interface

[krisbitney]

This PR updates the file system interface, adding additional features. The features were selected based on their inclusion in other file system APIs.

Details on the research can found in the issue comments: https://github.com/polywrap/std/issues/5

This PR introduces no breaking changes for AssemblyScript wrappers, but Rust wrappers will have issues due to missing None values where new optional method arguments are introduced. The following methods have additional options:

# original
mkdir(path: String!, recursive: Boolean): Boolean
# new
mkdir(path: String!, recursive: Boolean, mode: Int, mustCreate:  Boolean): Boolean

# original
rmdir(path: String!): Boolean
# new
rmdir(path: String!, recursive: Boolean): Boolean

Closes https://github.com/polywrap/std/issues/5