Implemented TryGetSection + minor changes.

Added `TryGetSection` methods to `Elf` and `IELF` classes. Reimplemented
associated `Get` methods to use TryGets.

Defined magic index `-1` with a meaninigful name `SymbolNameNotUniqueMarker` private constant.

Renamed template parameter in `GetSections` from `S` to `TSections`.