[Sec]: Zeroize the memory for each TWString

[satoshiotomakan]

Description

Zeroize the memory for each TWString and for the mnemonic data.

How to test

Run C++, iOS, Android tests.
Use memory dump tools to make sure if the mnemonic does not present in the memory.

Types of changes

Checklist

• Create pull request as draft initially, unless its complete.
• Add tests to cover changes as needed.
• Update documentation as needed.
• If there is a related Issue, mention it in the description.

If you're adding a new blockchain

• I have read the guidelines for adding a new blockchain.