A lot of code assumes that string_view::iterator is const char*. Does not work on every compiler

[hzeller]

A lot of code within Verible assumes that the iterator of string_view just decays to const char*. This assumption was true while absl::string_view was the implementation used, and even after switching to c++17, is as well true for gcc/clang standard library std::string_view.

However, on Windows, the iterator is its own wrapper object that can not be assigned to const char*, so Verible would fail to compile if we used that std::string_view there.

Thus, for a long while, we have a patch to tell abseil to always use the internal absl::string_view internal implementation on Windows and not just typedef absl::string_view to std::string_view.

However, due to that patch, our abseil-cpp in MODULES.bazel deviates from others users that depend on Verible in the registry. Not sure if this will be a problem, but it is an additional reason to get this fixed.

It would be good to remove these assumptions. This will probably involve using absl::string_view::iterator instead of const char* for parameters and data members where the intent is to work with iterators.

Also construction of std::string_view's that use iterators as start point will be different.

   absl::string_view foo0(bar.begin(), 42);  // before
   absl::string_view foo1(&*bar.begin(). 42);  // after (somewhat ugly)
   absl::string_view foo2(bar.data(), 42);  // slightly less ugly, but only works if we know it's begin.

This is a collection issue to collect all the pull requests towards that goal until everything is done.

Exit criteria: we can replace all absl::string_view with std::string_view and code compiles on all platforms.