api: cache imported roots for EDS

[Wondertan]

Currently, the EDS has a stateless design, meaning that each operation(repairing) with it recomputes and verifies the integrity of the data according to the given roots. Instead, we can cache the roots during import and stick to those during the whole lifespan of an EDS instance. This is especially useful for users who try to repair the same EDS multiple times over the same roots.

Proposed Changes:

• Remove roots params from Repair and cache them on EDS
• Don't reset roots in setCell together with api: Public SetCell  #83. Resetting was initially introduced to prevent users from breaking eds when using public setCell(which was actually never made). However, api: Public SetCell  #83 proposes another solution for proper encapsulation(setting only nil cell) which makes resetting obsolete.