Enable Redirecting Functions

Also enables background functions, but there's an issue with the shell hanging when a function is sent to te background

examples/function_piping.ion

@@ -17,3 +17,15 @@ fn format_with pat
 end
 
 echo one two three four five | format_with "-"
+
+fn square
+    read x
+    echo $(( x * x ))
+end
+
+fn mult x
+    read y
+    echo $(( y * x ))
+end
+
+echo 5 | square | mult 3

src/parser/pipelines/mod.rs

@@ -65,6 +65,13 @@ impl Pipeline {
             stdout.file = expand_string(stdout.file.as_str(), expanders, false).join(" ");
         }
     }
+
+    pub fn requires_piping(&self) -> bool {
+        self.jobs.len() > 1 ||
+            self.stdin != None ||
+            self.stdout != None ||
+            self.jobs.last().unwrap().kind == JobKind::Background
+    }
 }
 
 impl fmt::Display for Pipeline {

src/shell/mod.rs

@@ -180,7 +180,7 @@ impl<'a> Shell<'a> {
             builtins.get(key)
         } {
             // Run the 'main' of the command and set exit_status
-            if pipeline.jobs.len() == 1 && pipeline.stdin == None && pipeline.stdout == None {
+            if !pipeline.requires_piping() {
                 if self.flags & PRINT_COMMS != 0 { eprintln!("> {}", pipeline.to_string()); }
                 let borrowed = &pipeline.jobs[0].args;
                 let small: SmallVec<[&str; 4]> = borrowed.iter()
@@ -192,7 +192,7 @@ impl<'a> Shell<'a> {
             }
         // Branch else if -> input == shell function and set the exit_status
         } else if let Some(function) = self.functions.get(&pipeline.jobs[0].command).cloned() {
-            if pipeline.jobs.len() == 1 {
+            if !pipeline.requires_piping() {
                 let args: &[String] = pipeline.jobs[0].args.deref();
                 let args: Vec<&str> = args.iter().map(AsRef::as_ref).collect();
                 match function.execute(self, &args) {