Add port of libstd's path module to Theseus (#487)

* Currently, most fs-dependent parts are disabled

* don't include path_std by default

Author: kevinaboos

ports/path_std/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+authors = ["Kevin Boos <kevinaboos@gmail.com>"]
+name = "path_std"
+description = "Theseus port of the Rust std lib `path` module"
+version = "0.1.0"
+build = "../../build.rs"
+
+[dependencies]
+theseus_task = { path = "../../kernel/task", package = "task" }
+core2 = { version = "0.3.2", default-features = false, features = ["alloc", "nightly"] }