Support Rust toolchain from WSL

[tyt2y3]

I am using rust inside Windows Subsystem for Linux (WSL).
It'd be great if I can use the same toolchain to perform syntax checking.
After some fiddling, I found some success. 2 things are concerned.

1. finding the path to executable
2. converting file paths from /mnt/c/ to C:\\

I made a dirty patch to rust_proc.py:

diff --git a/rust_proc.py.bak b/rust_proc.py
index 5d650d1..503ed3e 100755
--- a/rust_proc.py.bak
+++ b/rust_proc.py
@@ -212,6 +212,8 @@ class RustProc(object):
             # Prevent a console window from popping up.
             startupinfo = subprocess.STARTUPINFO()
             startupinfo.dwFlags |= subprocess.STARTF_USESHOWWINDOW
+            self.cmd.insert(0, 'C:\\Windows\\System32\\wsl.exe')
+            self.cmd[1] = '/home/user/.cargo/bin/' + self.cmd[1]
             self.proc = subprocess.Popen(
                 self.cmd,
                 cwd=self.cwd,
@@ -298,6 +300,8 @@ class RustProc(object):
                 continue
             if self.decode_json and line.startswith('{'):
                 try:
+                    if sys.platform == 'win32':
+                        line = line.replace('/mnt/c/', 'C:\\\\')
                     result = json.loads(line)
                 except:
                     self.listener.on_error(self,

Now that syntax check is working. Not sure about other commands.