Add additional semantic-checking passes over the AST

[abhimadan]

We currently do some simple type-checking while constructing the AST, but some things we can't check include:

• variable scoping
• different type restrictions for different types of operators (e.g. different constraints for multiplication vs addition)

We can add a pass for each type of check. This will make the library slower at runtime, but won't be a problem once we transition to a DSL that's compiled with webpack.