Lazily import schemes to save startuptime

[robertwb]

This is a relatively self-contained target.

Split off of #8254

Component: misc

Keywords: sd32

Branch/Commit: public/ticket/11040 @ 8c82533

Issue created by migration from https://trac.sagemath.org/ticket/11040

[robertwb]

Attachment: 11040-lazy-schemes.patch.gz

[robertwb]

comment:2

Lazily importing functions, modules, constructors works fine. Lazily importing actual object that get passed around and stored has issues that need to be addressed.

[williamstein]

Changed keywords from none to sd32