message: Allow mapdir aliases with starting `/`
  Stripping leading `/` is valid because we mount mapped dirs at `/` regardless.
diff: @@ -438,7 +438,17 @@ impl PreopenDirBuilder {
  /// Make this preopened directory appear to the WASI program as `alias`
  pub fn alias(&mut self, alias: &str) -> &mut Self {
  - self.alias = Some(alias.to_string());
  + let alias = if let Some(first_byte) = alias.as_bytes().first() {
  + if *first_byte == b'/' {
  + &alias[1..]
  + } else {
  + alias
  + }
  + } else {
  + alias
  + }
  + .to_string();
  + self.alias = Some(alias);
  self
  }
  