string_view: implement iterator API for string_view

[KFilipek]

This PR introduce iterator API for string_view class.

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This change is 

[codecov]

Codecov Report

Merging #948 into master will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #948      +/-   ##
==========================================
+ Coverage   95.94%   96.17%   +0.23%     
==========================================
  Files          48       48              
  Lines        6138     6146       +8     
==========================================
+ Hits         5889     5911      +22     
+ Misses        249      235      -14     

Flag	Coverage Δ
#tests_clang_debug_cpp17	96.02% <ø> (+0.18%)	⬆️
#tests_gcc_debug	92.57% <100.00%> (+0.20%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
include/libpmemobj++/string_view.hpp	100.00% <100.00%> (ø)
...j++/container/detail/concurrent_skip_list_impl.hpp	98.12% <0.00%> (+0.13%)	⬆️
...ude/libpmemobj++/container/concurrent_hash_map.hpp	94.57% <0.00%> (+1.10%)	⬆️
include/libpmemobj++/mutex.hpp	86.20% <0.00%> (+6.89%)	⬆️

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[igchor]

Reviewed 3 of 4 files at r1.
Reviewable status: 3 of 4 files reviewed, 6 unresolved discussions (waiting on @KFilipek)

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tests/external/libcxx/string.view/string.view.iterators/begin.pass.cpp, line 8 at r1 (raw file):

//
//===----------------------------------------------------------------------===//

Intel license

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tests/external/libcxx/string.view/string.view.iterators/begin.pass.cpp, line 52 at r1 (raw file):

	test(string_view("123"));
	test(wstring_view(L"123"));
#if TEST_STD_VER >= 11

ifdef not needed

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tests/external/libcxx/string.view/string.view.iterators/begin.pass.cpp, line 57 at r1 (raw file):

#endif

#if TEST_STD_VER > 11

.

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tests/external/libcxx/string.view/string.view.iterators/end.pass.cpp, line 8 at r1 (raw file):

//
//===----------------------------------------------------------------------===//

.

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tests/external/libcxx/string.view/string.view.iterators/end.pass.cpp, line 50 at r1 (raw file):

{
	typedef pmem::obj::string_view string_view;
#if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L

not needed?

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tests/external/libcxx/string.view/string.view.iterators/end.pass.cpp, line 71 at r1 (raw file):

#endif

#if TEST_STD_VER > 11

.

[KFilipek]

Reviewed 3 of 3 files at r3.
Reviewable status: all files reviewed, 6 unresolved discussions (waiting on @igchor)

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tests/external/libcxx/string.view/string.view.iterators/begin.pass.cpp, line 8 at r1 (raw file):

Previously, igchor (Igor Chorążewicz) wrote…

Intel license

Done.

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tests/external/libcxx/string.view/string.view.iterators/begin.pass.cpp, line 52 at r1 (raw file):

Previously, igchor (Igor Chorążewicz) wrote…

ifdef not needed

Done.

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tests/external/libcxx/string.view/string.view.iterators/begin.pass.cpp, line 57 at r1 (raw file):

Previously, igchor (Igor Chorążewicz) wrote…

.

Done.

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tests/external/libcxx/string.view/string.view.iterators/end.pass.cpp, line 8 at r1 (raw file):

Previously, igchor (Igor Chorążewicz) wrote…

.

Done.

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tests/external/libcxx/string.view/string.view.iterators/end.pass.cpp, line 50 at r1 (raw file):

Previously, igchor (Igor Chorążewicz) wrote…

not needed?

Done.

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tests/external/libcxx/string.view/string.view.iterators/end.pass.cpp, line 71 at r1 (raw file):

Previously, igchor (Igor Chorążewicz) wrote…

.

Done.

[lukaszstolarczuk]

Reviewed 1 of 4 files at r1, 3 of 3 files at r3.
Reviewable status: all files reviewed, 6 unresolved discussions (waiting on @igchor)

[igchor]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @KFilipek)

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include/libpmemobj++/string_view.hpp, line 73 at r3 (raw file):

	constexpr const_iterator end() const noexcept;
	constexpr const_iterator cend() const noexcept;
#ifdef XXX

You can add support for it in this PR, it;s quite trivial

[igchor]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @KFilipek)