No Equals matcher for wide strings

**Description**
Currently there's no Equals matcher for wide strings.

**Additional context**
```
std::wstring test(L"test");
CHECK_THAT(test, Equals(L"test"));
```

error: no matching function for call to 'Equals(const wchar_t [5])'
CHECK_THAT(test, Equals(L"test"));
^
I'll look at adding this and raise a PR.