Editorial: Extract 'generatedPosition' and 'originalPosition' structs

[szuend]

This PR introduces two new structs generated position and original position and uses them in the decoded mapping struct instead of inlined line/column/sources. This has a couple of advantages:

• We make it explicit with the "types" that for original position in a mapping entry either all (source, line, column) are null or a non-negative integer. Otherwise it's only implicitly defined by the decoding algorithm.
• We can re-use both in the definition for "original scopes" (start, end) and "generated ranges" (start, end callsite).
• We can re-use both in various algorithms we'll declare (both for scopes and how to apply mappings).