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>

tests/arc.rs

@@ -1,11 +1,13 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
+#![allow(unused)]
 use lazy_static::lazy_static;
 use std::sync::Mutex;
 
 use regorus::*;
 
+#[cfg(feature = "arc")]
 // Ensure that types can be s
 lazy_static! {
     static ref VALUE: Value = Value::Null;
@@ -14,6 +16,7 @@ lazy_static! {
 }
 
 #[test]
+#[cfg(feature = "arc")]
 fn shared_engine() -> anyhow::Result<()> {
     let e_guard = ENGINE.lock();
     let mut engine = e_guard.expect("failed to lock engine");