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>

.github/workflows/pr.yml

@@ -22,6 +22,8 @@ jobs:
       run: sudo apt update && sudo apt install -y musl-tools
     - name: Format Check
       run: cargo fmt --check
+    - name: Build (all features)
+      run: cargo build -r --all-features --verbose
     - name: Build
       run: cargo build -r --verbose
     - name: Doc Tests