Expose memory usage

[bobrik]

Currently Regex is this struct:

pub struct Regex {
    pub(crate) meta: meta::Regex,
    pub(crate) pattern: Arc<str>,
}

Most of the methods proxy to self.meta methods. One notable exception is memory_usage, which exists in meta::Regex, but is missing on the public struct. It would be nice to have it for accounting purposes.

[BurntSushi]

Duplicate of #943, #1180

See also #1264