Fix semantic analysis

[mdbrnowski]

There are still a few things left to do, in particular handling for loops and variable scopes.

Originally posted by @mdbrnowski in #1 (comment)

• ranges and for loops (Fix ranges and for loops #5)
• compound assignments (Fix compound assignments #4)
• variable reassignments - during assignment and for loop (Fix redeclaration #6)
• better type handling (like during interpretation) (New semantic analysis #12)
• variable scopes (New semantic analysis #12)