Add support for volumes

[Michael-F-Bryan]

At the moment, wasmer-pack completely ignores volumes in the *.webc file you are generating code for. That means things like the python CLI won't work because they don't have access to the standard library.

To fix this, we need a couple things:

• Copy the volume's contents into the generated package (maybe as a *.tar.gz file?)
• Generate code that loads the volume file into a FileSystem object
• Set up the generated code so this FileSystem is mounted into the WASI instance