Dynamic hashing

[mitschabaude]

continuing from #62, this expands the scope of dynamic-schema-friendly hashing,

• make nested records work correctly (inner records are hashed dynamically as well)
• hash dynamic arrays - in particular, strings - such that the hash only depends on the actual elements (not the padding)
• hash primitive JS types (number, boolean, undefined, ...)
• general-purpose hashDynamic() and packToField() methods that work basically on anything, in a way that is compatible with DynamicArray.hash() (for arrays / strings) and DynamicRecord.hash() (for objects)

mostly finishes #68

TODOs:

• tests for complicated dynamic arrays
• tests for subschemas of nested records
• hash plain arrays such that it's compatible with dynamic arrays
• assertEquals(), assertStrictlyEquals() for DynamicArray
• easy and well-defined schema definition API
  • including generic strings, bytes and other arrays without specifying max size
  • left for next PR