Clean-up types and introduce runtime checks

[vgrichina]

We often don't have runtime checks for types "enforced" by TypeScript typings, see e.g. https://github.com/near/near-api-js/pull/452/files/777229d6bfa9b27ee21f5409dd74a686645786d0#r527117530

We need to introduce easy way to do such checks at scale.

There is also somewhat related issue of types not being correct in the first place. E.g. most places where BN is declared as type is actually accepting anything that can be converted to BN (i.e. BN, string and even small enough number). We should make this explicit.