Add iced, usfx, and assets-manager

[PatrickHoward]

Part of #111

μsfx - 0.1.3

μsfx is a small library built for generating sound effects in code during
runtime. μsfx can be integrated with the cpal and music crates as well as
SDL2 crate bindings.

Samples are available on μsfx's github readme and further
documentation is avialable on the docs.rs page.

This latest version (0.1.3) provides fixes for saw, triangle,
and square waves.

iced - v0.1

Iced is experimental, cross-platform GUI crate focusedd on simplicity and
type safety. Iced can be used natively, in a web browser, or can use wgpu,

The new release includes:

• Styling based on trait implementations.
• Event subscriptions that take place asynchronously by using streams.
• Canvas widgets, for drawing 2D graphics
• PaneGrid widgets, which can dynamically organize layout by splitting
  panes that can be resized.
• Svg widgets, which can render vector graphics.
• ProgressBar widgets
• Integration into exisiting wgpu projects.
• Options for integrating futures executors into a project.
• TextInput selection
• Texture Atlas support for iced-wgpu

Full docuementation for Iced is available on the docs.rs page
as well as examples and how to contribute are available on iced github repo

assets_manager - v0.2.2

assets_manager is a crate that provides convenient loading,
caching, and reloading of external resources. The crate is pay-for-what-you-take,
provides a high level API, and is concurrent.

This newest version provides for hot-reloading, directory-loading (being able
to load from a single directory all at once, for extensible games), meta
loaders, and various other improvements.

Examples and documentation are available on assets_manager's docs.rs
and additional information about contributing are available on the github repository

[ozkriff]

yay! thanks for the PR!