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/value.rs

@@ -406,6 +406,7 @@ impl Value {
 
     /// Deserialize a value from a file containing YAML.
     /// Note: Deserialization from YAML does not support arbitrary precision numbers.
+    #[cfg(feature = "std")]
     #[cfg(feature = "yaml")]
     pub fn from_yaml_file(path: &String) -> Result<Value> {
         match std::fs::read_to_string(path) {