Update glue js/c

[altafan]

In #85 we are changing the zkp interface, which is an effort to improve the usability, maintainability and readability of the this secp256k1-zkp lib.

Along with #31, we must also update the js/c glue to be compliant with the new interface:

The main changes involve:

• renaming Ec as Ecc and declaring it as extension of tiny-secp interface
• replacing Buffer with Uint8Array for browser support
• replacing number with string to prevent any conversion issue from js to c
• removing serialize/parse methods (implementation details that should be hided in out abstraction)
• representing surjection proof as Uint8Array instead of object with low-level details
• renaming and re-ordering some methods' args for readability

[altafan]

Closed by #33..#37