Iostreams: Support filesystem3 paths. Refs #4485.

Based on Zhuo Qiang's patch with added tests and support
for `operator=`.

I also used `codecvt_type` to detect `filesystem3::path`
instead of `string_type`. Using `string_type` made
`filesystem2::path` ambiguous because it has both
`string_type` and `external_string_type` members. 

The member types are a bit arbitrary, but a more precise
distinction would probably also be more expensive.



[SVN r72382]

1 files changed, 0 insertions, 0 deletions

diff --git a/libs/iostreams b/libs/iostreams
-Subproject f05a47d0a1d124907b5c690fd60b5ed87351b5b
+Subproject 0ef80870404ac129ebe5dca8e319c6766cf2cb6