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/engine.rs

@@ -10,7 +10,7 @@ use crate::utils::gather_functions;
 use crate::value::*;
 use crate::{Extension, QueryResults};
 
-use std::convert::AsRef;
+use core::convert::AsRef;
 use std::path::Path;
 
 use anyhow::{bail, Result};