Add to_bytes to SecretKey

[SebastienGllmt]

I needed this for debugging. Figured it might be useful for somebody else in the future.

[coveralls]

Pull Request Test Coverage Report for Build 962

• 8 of 9 (88.89%) changed or added relevant lines in 2 files are covered.
• 75 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.1%) to 87.056%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergo-lib/src/wallet/secret_key.rs	6	7	85.71%

Files with Coverage Reduction	New Missed Lines	%
ergo-lib/src/ast.rs	5	37.5%
ergo-lib/src/ergo_tree.rs	13	90.48%
ergo-lib/src/chain/address.rs	23	76.72%
ergo-lib/src/ast/constant.rs	34	76.1%

Totals
Change from base Build 929:	0.1%
Covered Lines:	2596
Relevant Lines:	2982

---

💛 - Coveralls

[greenhat]

Thanks! Looking good! Please fix formatting (red CI).