if size_t_is_usize is set to true, and bindgen handles a size_t, and size_t ≠ uintptr_t, bindgen should abort

Many projects (including nettle-sys, linux-kernel-module-rust, nodejs-sys, bcc-sys) are moving to revert to the pre-0.53 behavior that maps size_t and usize. These projects (and many others if #1902 is merged) will fail subtly if they use bindgen over a C API that includes size_t on a weird niche platform where size_t ≠ uintptr_t. if bindgen detects that this situation is happening, it should produce an error.

the three conditions that should produce an error (all conditions must be present) are:

• size_t_is_usize is set to true
• The source C API includes a size_t
• The platform's size_t is not the same underlying type as uintptr_t

(see also #1901 and #1671 for background)

Start by reviewing bindgen's size_t_is_usize option and its size_t handling, then read issues #1902, #1901, and #1671 for the surrounding behavior. Done means bindgen reports an error only when the option is true, the C API uses size_t, and size_t differs from uintptr_t on the target platform.