Add PreservedStrExpr utility wrapper - #346

By default, darling evaluates the expression inside string literals. This can be worked around with preserve_str_literal, but sometimes it's difficult—for instance, when a struct contains HashMap<syn::Ident, syn::Expr>.

This commit introduces a utility wrapper over syn::Expr that has a simple FromMeta impl that just copies the inner Expr instance. It can be then used like so:

#[derive(FromMeta)]
#[darling(attributes(demo))]
struct Demo {
    option: Option<HashMap<syn::Ident, PreservedStrExpr>>,
}