Add if-then-else statement support

[stringhandler]

Built on #185, so that would need to be merged first.

Addresses #181

[gerau]

Could we make the else expression optional? Maybe we could evaluate a missing else as the unit type (), so this would prevent conflicts when using an if statement in an assignment, for example.

[KyrylR]

#185 is merged, so you can rebase

[uncomputable]

This is an if-then-else expression, by the way.

[apoelstra]

Sorry for the long delay on this. I thought it would be much harder to review than it was, and I had a big pile of notifications to climb out from under. I should be available closer to real-time from here on out.

In ddf467b:

Are we intentionally allowing expressions that are not blocks? For example tests still pass with this diff:

diff --git a/src/ast.rs b/src/ast.rs
index 3c32866..0ca53ba 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -1733,7 +1733,7 @@ mod test {

     #[test]
     fn test_if_expression_with_complex_arms() {
-        let input = "if false { let x: u8 = 5; x } else { 10 }";
+        let input = "if false { let x: u8 = 5; x } else 10";

         let parsed_expr = parse::Expression::parse_from_str(input).expect("Failed to parse");

This seems okay to me, but it does differ from Rust and if it's intentional we should call it out.

[stringhandler]

Oh interesting. I thought that was valid in Rust, but it explicitly requires blocks to avoid mistakes. I think it's worth sticking with Rust's rule of requiring brackets. I'll update it

[stringhandler]

rebased with brace validation