Ab/make parentchain pallet instantiable

[brenzi]

closes #227

• make pallet parentchain instantiable multiple times (once for each parentchain)
• introduce ShardVault storage: if this is set, the parentchain should be considered a target for shielding/unshielding
• introduce account mirror: host a double map for parentchain AccountInfo
• introduce parentchain genesis (if we don't store (and then verify) this with the shard state, an attacker could exchange light client db's)