Core language

[julianhyde]

Add a "core" language that is intermediate between Ast and Code/Applicable.

The elements of Core look similar to the expressions in Ast, but are simpler/fewer, and are all explicitly typed.

Advantages:

• Core should allow us to do some optimizations, for example inlining constants.
• Smaller language means fewer cases to consider.
• Included types mean that we won't hit missing types all the time. (Currently we have to remember to add newly generated Ast elements to Compiler.typeMap.)

[julianhyde]

Fixed in b72f1d9.