pull size_t from the std namespace

[qbojj]

Fix #2190

I have decided to use std::size_t in nonstd/string_view.hpp as there were already parts where it was used

[lemire]

Please do not modify the string_view code. It is not simdjson code, it is merely bundled for convenience. If you have concerns about about the string_view implementation, please first take it with @martinmoene as we are not going to manually modify their code:
https://github.com/martinmoene/string-view-lite

[qbojj]

I have reverted the changes to string_view

[lemire]

Running tests.

[lemire]

Merging.