Extract implementation of yew-macro in it's own crate

[hamza1311]

Problem

yew-macro crate currently contains the parser and implementation of html! and other macros. It is a proc-macro crate which means it can't be added as a dependency (or unit tested)

Solution

Create a new crate that is not a proc-macro crate that exports the parsing and implementation details of the macros. yew-macro depends on it and uses it to expand the macros.
This also allows the playground to expand html! macro and show it's output without invoking rustc.
This crate may also expanded to become an alternative to syn-rsx

Drawbacks

It is an additional crate to publish.

[filipdutescu]

I could really use this separation, since I would like to add some default properties to my components, which currently require a bunch of copy pasting. Hopefully having access to the html! macro inners would make it much more pleasant to do. Is there any progress on this, by any chance?