For context, String, Nil, and Bool already behave this way.
Note: Before it used to construct (call, ... (integer_literal)), and the
call would be made explicit / implicit based on if you did eg: Int(3) or
just 3. This however did not translate to the new world so this PR adds
a IsExplicitConversion bit to NumberLiteralExpr. Some side results of
all this are that some warnings changed a little and some instructions are
emitted in a different order.