P3620 R0 Concerns with the proposed addition of fibers to C++ 26

[wg21bot]

P3620R0 Concerns with the proposed addition of fibers to C++ 26 (David Chisnall, Matthew Taylor)

[jensmaurer]

Design concerns should be handled by EWG. Removing CWG and LWG.

[jfbastien]

Related to #117
Note: fibers missing C++26.