Tokenize the input with flex

TODO

@@ -1,20 +1,14 @@
 # Compiler
 
-- Whitespaces are checked twice, in the source class and by lexing.
 - Catch exceptions thrown by the argument parser and print them normally.
-- Parser should be able to collect errors.
 - Provide position information on parse tree nodes.
-- Move constants to the symbol table, so we can check at parse time for duplicates.
-- Don't pass raw pointers to the visitor methods.
 - While loop.
 - Type checking.
-- Procedures.
 - Calculate additional stack space needed for subexpressions in the allocator
   visitor and not in the backend.
 - Support immediates greater than 12 bits.
 - It seems instructions are correctly encoded only if the compiler is running
   on a little endian architecture.
-- Merge declaration and definition nodes.
 - Pointer.
 - Static array.