Add wire.Outpoint parser from string

[GeorgeTsagk]

• Adds NewOutPointFromString function in wire package.
• Adds unit tests for NewOutPointFromString

[guggero]

Nice feature! Definitely nice to have that in the wire package directly.

[GeorgeTsagk]

@guggero
Also added a new test case named big index outpoint, which includes a tx index which is a bit higher than the max int32 value.

[guggero]

LGTM 🎉

[bhandras]

Just two small nits remain, otherwise LGTM 🎉

[coveralls]

Pull Request Test Coverage Report for Build 2868576184

• 13 of 17 (76.47%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 55.143%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wire/msgtx.go	13	17	76.47%

Files with Coverage Reduction	New Missed Lines	%
btcutil/gcs/gcs.go	1	81.25%

Totals
Change from base Build 2836573928:	0.01%
Covered Lines:	26533
Relevant Lines:	48117

---

💛 - Coveralls

[Roasbeef]

LGTM 🦖

[coveralls]

Pull Request Test Coverage Report for Build 2876839350

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 13 of 17 (76.47%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 55.139%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wire/msgtx.go	13	17	76.47%

Files with Coverage Reduction	New Missed Lines	%
btcutil/gcs/gcs.go	1	81.25%

Totals
Change from base Build 2836573928:	0.01%
Covered Lines:	26531
Relevant Lines:	48117

---

💛 - Coveralls