(PUP-2027) Support lambdas in 4x function API

• Adds ability to accept lambdas/blocks in the 4x function API
• Adds ability to define "last arg captures rest"
• Adds CallableSignature as API for information about a callable (was different for functions and lambdas earlier)
• Function and Closure now implements CallableSignature
• Refactoring of how evaluator calls functions (no need to assert

function exsistnce separately (just takes time to look up functions
twice).

• TypeCalculator can now infer Callable from Function and Closure
• TypeFactory supports creating Callable with a block type last
• Callable type is given convenience methods to get range of types and block (accepted, optional, required).
• Spec tests added for many additional examples (for blocks)