Fix VS2017 string_view support

[alabuzhev]

I've tried to address #657 here by encapsulating the iterator type.

I'm not sure if it's the best possible approach, but at least it works.

Note: cmake scripts really should be updated to add /std:c++latest compiler option to enable the "real" VS17 string_view - right now this path is not tested, which is presumably why this issue was not identified earlier.

[alabuzhev]

VS17 asserts that iterators must point to the same string_views:

constexpr difference_type operator-(const _String_view_iterator& _Right) const _NOEXCEPT
{	// return difference of iterators
#if _ITERATOR_DEBUG_LEVEL >= 1
	_IDL_VERIFY(_Mydata == _Right._Mydata && _Mysize == _Right._Mysize,
		"cannot subtract incompatible string_view iterators");

After remove_prefix both format_str_'s data and size will be different and this check won't pass.
I've replaced format_str_ with a pair of string_view iterators to resolve this.

Note: it still might fire somewhere since end_ will not be updated and will point to a 'different' string_view after advance_to. I was not able to reproduce it, but it probably should be checked more thoroughly.

[vitaut]

Fixed by using fallback string_view (#664), but thanks anyway.