Add WSTRING support for structures

[stlehmann]

Targets #291

[coveralls]

Pull Request Test Coverage Report for Build 1576787639

• 36 of 36 (100.0%) changed or added relevant lines in 3 files are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 94.255%

---

Files with Coverage Reduction	New Missed Lines	%
pyads/pyads_ex.py	1	90.37%
pyads/testserver/testserver.py	5	90.76%
<!--	Total:	6		-->

Totals
Change from base Build 1519582091:	-0.2%
Covered Lines:	1690
Relevant Lines:	1793

---

💛 - Coveralls

[stlehmann]

This adds support for WSTRINGs in structures, both as fields and as arrays. @chrisbeardy would you mind taking a look at this PR and giving a short review as this involves some changes on the codebase written by you.