add `std` feature

[anakrish]

• std feature is enabled by default
• By default enable #![no_std] compilation
• Import std create if std feature is enabled or if testing
• Use core, alloc types
• Make it clear where std types are being used
• In no std, use BTreeMap in place of HashMap. HashMap is not available in no std due to lack of a secure random number generator

Note: The project does not yet compile without std feature being specified. But it's really close to being able to do so.

[anakrish]

Merging to make progress.