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>

src/engine.rs

@@ -8,11 +8,9 @@ use crate::parser::*;
 use crate::scheduler::*;
 use crate::utils::gather_functions;
 use crate::value::*;
+use crate::*;
 use crate::{Extension, QueryResults};
 
-use core::convert::AsRef;
-use std::path::Path;
-
 use anyhow::{bail, Result};
 
 /// The Rego evaluation engine.
@@ -89,7 +87,8 @@ impl Engine {
     /// # Ok(())
     /// # }
     /// ```
-    pub fn add_policy_from_file<P: AsRef<Path>>(&mut self, path: P) -> Result<()> {
+    #[cfg(feature = "std")]
+    pub fn add_policy_from_file<P: AsRef<std::path::Path>>(&mut self, path: P) -> Result<()> {
         let source = Source::from_file(path)?;
         let mut parser = Parser::new(&source)?;
         self.modules.push(Ref::new(parser.parse()?));