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/number.rs

@@ -1,9 +1,9 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
+use alloc::str::FromStr;
+use core::cmp::{Ord, Ordering};
 use core::fmt::{Debug, Formatter};
-use std::cmp::{Ord, Ordering};
-use std::str::FromStr;
 
 use anyhow::{anyhow, bail, Result};
 
@@ -61,7 +61,7 @@ pub enum Number {
 }
 
 impl Debug for Number {
-    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), std::fmt::Error> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), core::fmt::Error> {
         match self {
             Number::Big(b) => b.d.fmt(f),
         }