Avoid dependency on `source lifetime. (#43)

This allows holding onto objects, caching results etc easily.
However it does introduce the overhead of ref counting.

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

src/builtins/debugging.rs

@@ -1,7 +1,7 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
-use crate::ast::Expr;
+use crate::ast::{Expr, Ref};
 use crate::builtins;
 use crate::lexer::Span;
 use crate::value::Value;
@@ -20,7 +20,7 @@ pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
 // Symbol analyzer must ensure that vars used by print are defined before
 // the print statement. Scheduler must ensure the above constraint.
 // Additionally interpreter must allow undefined inputs to print.
-fn print(span: &Span, _params: &[Expr], args: &[Value]) -> Result<Value> {
+fn print(span: &Span, _params: &[Ref<Expr>], args: &[Value]) -> Result<Value> {
     if args.len() > MAX_ARGS as usize {
         bail!(span.error("print supports up to 100 arguments"));
     }