Add <Symbol> component (#62)

* Use child and svg::InertElement instead of inner_html for speed improvements.

* Adds a <Symbol> element for rendering optimization. Adds an example for <Symbol>.

* Use String for <Symbol> id instead of &'static str.

* update: Cargo.lock

* update: examples Cargo.lock

* Fix doc test.

Fix small typo in `<Symbol>` doc test.

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Co-authored-by: Charles Edward Gagnon <charlesedgagnon@gmail.com>

Author: bicarlsen