fix(base): copy a const Function instead of wrapping it

[Force67]

Function's converting constructor excluded Function itself but not const Function. Moving a closure that captured a Function by copy moves that capture as a const Function&&, which bound to the converting constructor ahead of the copy constructor and wrapped the Function in another Function, and again, until the stack ran out. rx hits this in Renderer::BuildFrameGraph for every scene hook (the pass lambda captures the hook by copy); anyvoxel crashed on its first frame.

StaticFunction's converting constructor and assignment had no constraint at all, so copying a non-const StaticFunction lvalue failed its storage static_assert.

• Function: IsCallable strips cv-qualifiers (remove_cvref_t)
• StaticFunction: converting constructor and assignment reject StaticFunction and nullptr
• tests: Function from a const rvalue and from a moved closure that captured one (both crash before the fix); StaticFunction copy from a non-const lvalue and a const rvalue (did not compile before)

base_unittests 1005/1005.