Check assignment type

TODO

@@ -3,7 +3,6 @@
 - Catch exceptions thrown by the argument parser and print them normally.
 - Structs or records.
 - Unions.
-- Type checking.
 - Support immediates greater than 12 bits.
 - It seems instructions are correctly encoded only if the compiler is running
   on a little endian architecture.
@@ -13,5 +12,3 @@
 - Error message with an empty file wrongly says that a ")" is expected.
 - Support any expressions for constants.
 - Name analysis should fail if there are undefined symbols.
-- type_mismatch error should get second type argument for the
-  expected type.