Add support for including slices of binary files

Adds 2 new forms of `incbin`:

- `incbin(relative_filename, start)`: includes `relative_filename` starting from offset `start`
- `incbin(relative_filename, start, size)`: includes `size` bytes from `relative_filename` starting from offset `start`

I'd also like to make `incbinstr` and `inchexstr` work similarly, but I'm not sure how to make it work.

When hlorenzi#191 gets merged, `ensure_inc_args` can be replaced with a call to `query.ensure_min_max_arg_number(1,3)?;`. Since I don't have that merged locally, I just wrote a quick and dirty function to do it.

src/asm/resolver/eval_fn.rs

@@ -115,6 +115,26 @@ pub fn eval_fn(
 }
 
 
+fn ensure_inc_args(
+    query: &mut expr::EvalFunctionQuery
+) -> Result<(),()>
+{
+    if query.args.len() < 1 || query.args.len() > 3
+    {
+        query.report.error_span(
+            format!("function expected 1 to 3 arguments (got {})", query.args.len()),
+            query.span
+        );
+
+        Err(())
+    }
+    else
+    {
+        Ok(())
+    }
+}
+
+
 fn eval_builtin_incbin(
     fileserver: &mut dyn util::FileServer,
     _decls: &asm::ItemDecls,
@@ -123,7 +143,7 @@ fn eval_builtin_incbin(
     query: &mut expr::EvalFunctionQuery)
     -> Result<expr::Value, ()>
 {
-    query.ensure_arg_number(1)?;
+    ensure_inc_args(query)?;
 
     let relative_filename = query.args[0].value.expect_string(
         query.report,
@@ -148,8 +168,53 @@ fn eval_builtin_incbin(
         Some(query.args[0].span),
         file_handle)?;
 
-    Ok(expr::Value::make_integer(
-        util::BigInt::from_bytes_be(&bytes)))
+    if query.args.len() == 1 {
+        Ok(expr::Value::make_integer(
+            util::BigInt::from_bytes_be(&bytes)))
+    } else if query.args.len() == 2 {
+        let start = query.args[1].value.expect_usize(query.report, query.args[1].span)?;
+
+        if start >= bytes.len() {
+            query.report.error_span(
+                format!("incbin starts after EOF ({} > {})",
+                        start,
+                        bytes.len()),
+                query.args[1].span
+            );
+            return Err(());
+        }
+
+        Ok(expr::Value::make_integer(
+            util::BigInt::from_bytes_be(&bytes[start..bytes.len()])))
+    } else {
+        let start = query.args[1].value.expect_usize(query.report, query.args[1].span)?;
+
+        let size = query.args[2].value.expect_usize(query.report, query.args[2].span)?;
+
+        if start >= bytes.len() {
+            query.report.error_span(
+                format!("incbin starts after EOF ({} > {})",
+                        start,
+                        bytes.len()),
+                query.args[1].span
+            );
+            return Err(());
+        }
+
+        if (start+size) > bytes.len() {
+            query.report.error_span(
+                format!("incbin ends after EOF ({} + {} > {})",
+                        start,
+                        size,
+                        bytes.len()),
+                query.args[2].span
+            );
+            return Err(());
+        }
+
+        Ok(expr::Value::make_integer(
+            util::BigInt::from_bytes_be(&bytes[start..(start+size)])))
+    }
 }
 
 
@@ -259,4 +324,4 @@ fn eval_builtin_incstr(
     }
 
     Ok(expr::Value::make_integer(bitvec.to_bigint()))
-}
+}