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/utils.rs

@@ -5,7 +5,7 @@ use crate::ast::*;
 use crate::builtins::*;
 use crate::lexer::*;
 
-use std::collections::BTreeMap;
+use alloc::collections::BTreeMap;
 
 use anyhow::{bail, Result};
 pub fn get_path_string(refr: &Expr, document: Option<&str>) -> Result<String> {