use & instead of start-process in x.ps1

start-process has weird parsing rules and buggy behavior. we've already had to work around them several times, and the workarounds were not complete.
i wonder who could have added it HMMMMMM
```
PS C:\Users\jyn\src\rust> git log --reverse -S Start-Process x.ps1
commit 775c3c0493
Author: Jynn Nelson <github@jyn.dev>
Date:   Sun Jul 31 14:02:31 2022 -0500

    Add `x.sh` and `x.ps1` shell scripts
```

the latest broken thing is trailing backslashes:
```
$ x.ps1 t .\tests\ui\error-emitter\
```
would be transformed into
```
['t', '.\\tests\\ui\\error-emitter"']
```

rather than trying to hack around that too, abandon start-process altogether and just use `&`.

x.ps1

@@ -8,12 +8,7 @@ $ErrorActionPreference = "Stop"
 Get-Command -syntax ${PSCommandPath} >$null
 
 $xpy = Join-Path $PSScriptRoot x.py
-# Start-Process for some reason splits arguments on spaces. (Isn't powershell supposed to be simpler than bash?)
+$xpy_args = @($xpy) + $args
-# Double-quote all the arguments so it doesn't do that.
-$xpy_args = @("""$xpy""")
-foreach ($arg in $args) {
-    $xpy_args += """$arg"""
-}
 
 function Get-Application($app) {
     $cmd = Get-Command $app -ErrorAction SilentlyContinue -CommandType Application | Select-Object -First 1
@@ -21,16 +16,8 @@ function Get-Application($app) {
 }
 
 function Invoke-Application($application, $arguments) {
-    $process = Start-Process -NoNewWindow -PassThru $application $arguments
+    & $application $arguments
-    # WORKAROUND: Caching the handle is necessary to make ExitCode work.
+    Exit $LASTEXITCODE
-    # See https://stackoverflow.com/a/23797762
-    $handle = $process.Handle
-    $process.WaitForExit()
-    if ($null -eq $process.ExitCode) {
-        Write-Error "Unable to read the exit code"
-        Exit 1
-    }
-    Exit $process.ExitCode
 }
 
 foreach ($python in "py", "python3", "python", "python2") {