no_std support

[anakrish]

• 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.