Multiple methods for initializing analysis values

Introduces a number of methods that are called when initializing or
updating abstract values in an analyzer frame.

Before this commit, Analyzer.analyze and Frame.execute would always
call Interpreter.newValue for initializing or updating frame values.

Having multiple methods allows users to return more precise values
for the individual cases. For example, in a nullness analysis, the
initial value for the `this` parameter of an instance method can be
set to not-null.