Add Windows support to cross builder

[webmaster128]

This largely builds on #117 which can now be used with Wasmer 2.1.
With this we can build an api/wasmvm.dll.

Closes #117
Closes #28

[leejw51crypto]

unit test fails here by this reason
create_dir_all(path).map_err(|e| {
VmError::cache_err(format!(
"Error creating directory {}: {}",
path.display(),
e
))
})?;

1. path separator \\ -> / , like normalize_path(path.display())
2. probably ignore second {} or "Error creating directory {}"

[devashishdxt]

One issue I found is that loupe reports different module size on Windows and other OSes (here).

[webmaster128]

This work has been integrated in the meantime. See #389 for an overview of the state of Windows support.