Userspace fexec

Implements fork and execve here instead of in the kernel.

src/platform/redox/redox-exec/Cargo.toml

@@ -2,7 +2,7 @@
 name = "redox-exec"
 authors = ["4lDO2 <4lDO2@protonmail.com>"]
 version = "0.1.0"
-edition = "2018"
+edition = "2021"
 license = "MIT"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html