Re-thinking: Layering of configuration

We currently (as of 0.13.x) support layering of configuration objects to merge configuration values.

This should be supported in the future as well.

Re-Thinking the layering mechanism

We might re-think the idea of layering a bit here.
Right now (AFAICR) the layering is executed on loading of the configuration. In future, we might implement this as an operation that happens on accessing the values.
This way, we would always keep all values from the sources and only "merge" the values when accessing a certain element.

Example:

# 1.toml
foo = 1
bar = 2

# 2.toml
bar = 3

// pseudo code:
Build config object
Register "1.toml"
Register "2.toml"
Access config at "bar"
  - Mechanism finds out that "bar" is from "2.toml" and its value is 3

Start by locating the existing configuration-layering implementation and reading how configuration objects are loaded and accessed. Use the 1.toml and 2.toml example to define the intended behavior, including bar resolving to 3 while preserving the source values. Done means the future layering approach is specified and its behavior is covered by tests.