Remove methods that neither used nor export

[jasonandjay]

1. For Signer & Network, we have defined and exported it from psbt module.
2. For UInt31, BIP32Path and ECPoint, we have neither used nor exported them. The check of BIP32Path should be related to BIP32, and ECPoint should be related to ECPair. This project is not suitable for checking.

So I think it's time to remove this methods.

In addition, I suggest that we stop using typeforce to verify input parameters and use ts types instead for better maintenance.

[junderw]

use ts types instead

TS types don't check at runtime. Some people still use plain old JavaScript, and some people also use @ts-ignore etc. against all best practices because "it just works".

If anything, there are probably some functions that SHOULD have runtime type checks that don't, and we should add more.

That said, I have #2030 which I think is necessary.