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/time/compat.rs

@@ -34,7 +34,6 @@
 use crate::*;
 use core::fmt;
 use core::iter;
-use std::error::Error;
 
 use chrono::TimeZone;
 use chrono::{
@@ -72,8 +71,6 @@ impl fmt::Display for ParseDurationError {
     }
 }
 
-impl Error for ParseDurationError {}
-
 // Parses a duration string in the form of `10h12m45s`.
 //
 // Adapted from Go's `time.ParseDuration`: