Favor a single workspace with examples

[mciantyre]

It's faster to build all examples in a single package, rather than separate packages. Additionally, this lets us drop some of our extra CI steps which were combining features.

Remove the default build target; instead, build for your host. This makes it easier to use cargo test, and run unit tests on your host. When building examples, you'll need to specify the MCU target; see updated example docs for details.

Remove the Makefile, since it's no longer needed.

[mciantyre]

bors try

[bors[bot]]

try

Build failed:

• doc

[mciantyre]

bors try

[bors[bot]]

try

Build succeeded:

• format
• precompiled
• test
• doclinks
• clippy
• xplat (ubuntu-latest)
• xplat (macos-latest)
• xplat (windows-latest)
• template
• doc