add std feature (#231)

- `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.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

scripts/pre-push

@@ -11,6 +11,9 @@ if [ -f Cargo.toml ]; then
 
    # Ensure that the public API works
    cargo test -r --doc
+
+   # Ensure that we can build with all features
+   cargo build -r --all-features
    
    # Ensure that all tests pass
    cargo test -r