Use alloc, core instead of std (#225)

- Replace std with alloc, core in most places in src
  Tests, bindings aren't changed.
- Introduce BuiltinsMap type alias inplace of HashMap.
  In no_std case, this could be aliases to BTreeMap
- Fix clippy warnings

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

src/builtins/time/compat.rs

@@ -31,9 +31,9 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+use core::fmt;
+use core::iter;
 use std::error::Error;
-use std::fmt;
-use std::iter;
 
 use chrono::TimeZone;
 use chrono::{