no_std support (#232)

- Disable default features in dependencies
- Use anyhow::Error::msg to map errors. Note: anyhow will itself be removed later.
- lazy_static/spin_no_std used in no_std environments
- ensure_no_std binary is built to target  thumbv7m-none-eabi to ensure that
  there are no std dependencies.  thumbv7m-none-eabi target has no std support.
- The opa-no-std feature enables only those Regorus features that work with no_std.
- Enable tests with no_std
- Update sizes of regorus binary in  README.md
- Ensure that regorus example can be built with only std
- Ensure that regorus example can be built with no_std

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/builtins/mod.rs

@@ -9,7 +9,6 @@ mod conversions;
 
 #[cfg(feature = "crypto")]
 mod crypto;
-mod debugging;
 #[cfg(feature = "deprecated")]
 pub mod deprecated;
 mod encoding;
@@ -54,8 +53,6 @@ use lazy_static::lazy_static;
 
 pub type BuiltinFcn = (fn(&Span, &[Ref<Expr>], &[Value], bool) -> Result<Value>, u8);
 
-pub use debugging::print_to_string;
-
 #[cfg(feature = "deprecated")]
 pub use deprecated::DEPRECATED;
 
@@ -104,7 +101,6 @@ lazy_static! {
 	//rego::register(&mut m);
 	#[cfg(feature = "opa-runtime")]
 	opa::register(&mut m);
-	debugging::register(&mut m);
 	tracing::register(&mut m);
 	units::register(&mut m);