Bumps secp256k1 to v4

Changelog:

• #104

v0.5.5

Changes:

• Fixes bug in building BTC outputs. Previously we were naively adding a change output, even in cases where the change value was 0.

v0.5.4

Compatible with firmware v0.8.2

• Fixes bug in retry logic and updates all tests.

v0.5.3

Per small change in firmware, ETH transaction maximum data size has increased from 1000 to 1024. This affects the size of all ETH transactions and is a small breaking change.

v0.5.2

This is the first production release of the GridPlus SDK.

• Compatible with Lattice1 firmware version v0.8.2