toml.xml: single-quoted keys and a fir for integers

This MR adds support for single-quoted segments in keys like used in Cargo.toml for platform-specific deps in Rust projects.

[target.'cfg(all(windows, target_env="msvc"))'.build-dependencies]
vcpkg = "0.2"

Also contains a drive-by fix to make 0_123456 show up as an error (due to the leading zero).