Create a SymbolProcessor for generating Pullback boilerplate

[semanticer]

Description

This is a continuation of #60 (comment)

The idea is still to generate pullback methods as described in the original issue:

// This we can infer by checking that `SettingsReducer` implements `Reducer<SettingsState, SettingsAction>`
// and also by checking that both `SettingsState` and `SettingsAction` have generated action mappings.
fun Reducer<SettingsState, SettingsAction>.pullback() = pullback(
    ::mapAppStateToSettingsState,
    ::mapAppActionToSettingsAction,
    ::mapSettingsStateToAppState,
    ::mapSettingsActionToAppAction,
)

But the thing is more complicated that we thought, as described here.