internal/manifest: Introduce L0SubLevels data structure

This change introduces L0SubLevels, a data structure to encaptulate
all information to compute L0 Sublevels, flush split keys, and
base / intra-L0 compactions. This data structure will be generated
every time there's a file addition/deletion in L0, the integration
bits will come in future PRs.

The methods to pick base / intra-L0 compactions will come in a
separate PR.

Captures some of the largest code pieces of #563, and effectively
replaces a part of that PR. Sumeer wrote the bulk of this code as
part of his prototype, so thanks Sumeer for his work.

Most of these functions are dead code at the moment, only invoked
by the provided datadriven and non-datadriven tests.