Add syntactic sugar for installing nullary methods

[d-torrance]

As proof of concept, use it for all the nullary methods defined in Core.

For example:

i1 : foo = method()

o1 = foo

o1 : MethodFunction

i2 : foo () := () -> print "Hello, world!"

o2 = FunctionClosure[stdio:2:10-2:37]

o2 : FunctionClosure

i3 : foo()
Hello, world!

Inspired by #4090 (comment). I'm not sure why this wasn't implemented before. It's mentioned as a possibility in the docs, and it's literally just one line of code!