parser: Implement variable assignment

This commit introduces variable assignment making it possible to
change a variable value. Example:

  myvar: i32 = 1;
  myvar = 2;

Signed-off-by: Johnny Richard <johnny@johnnyrichard.com>
Co-authored-by: Carlos Maniero <carlos@maniero.me>

1 files changed, 4 insertions, 2 deletions

diff --git a/examples/variables.pipa b/examples/variables.pipa
index 25cbd59..0723782 100644
--- a/examples/variables.pipa
+++ b/examples/variables.pipa
@@ -1,6 +1,8 @@
 main(): i32 {
-  a: i32 = 12;
-  b: i32 = 32;
+  a: i32 = 13;
+  a = a - 1;
+  b: i32 = 33;
+  b = 32;
   c: i32 = 2 * (b + a);
   d: i32 = (c - 33) + 1;
   e: i32 = d;