Fix Ledger/Trezor transaction signing

[JamesLefrere]

This PR makes some changes to the purser-ledger and purser-trezor packages (necessitating some small changes to purser-core) such that the seeded signature values for the transaction to be signed (i.e. r, s and v) are not included. This is because ethereumjs-tx makes validations for EIP-155 that read these values (if present) and compare them with the chain ID, which was leading to validation errors for mainnet transactions.

Changes

• Remove initial r, s and v values from the unsigned transaction for Ledger and Trezor
• Add bigNumberToHexString util
• Update Ledger packages (minor version, no breaking changes)

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