Macro for constructing `duct::Expression`

[tobz1000]

This is something I've found myself wanting as I consider migrating a project from using std::process::Command to using duct::Expression. Unfortunately there's no easy way to convert between the two types, so the command! macro can't be used.

I've made an initial implementation & tests using proc_macro; all the functionality of the command! macro is intact, with the following changes:

• {x} takes a duct::Expression
• (x) takes Into<OsString>, as required by Expression, rather than AsRef<OsString>; as a result, the argument is moved instead of referenced
• [x] takes IntoIterator<Item=impl Info<OsStr>> rather than IntoIterator<Item=impl AsRef<OsStr>>

A slight rough edge is the error message if the user doesn't have the duct crate in dependencies (or it's been renamed) - this gives the error message "could not find duct in {{root}}". I'm not sure if it's possible to replace that message.

I'd be happy to update documentation and put up a PR if this would be wanted.