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/lexer/mod.rs

@@ -225,6 +225,7 @@ fn invalid_line() -> Result<()> {
 }
 
 #[test]
+#[cfg(feature = "std")]
 fn file_more_than_64_kb_size() -> Result<()> {
     let source = Source::from_file("tests/kata/data/large.rego")?;
     let mut lexer = Lexer::new(&source);