add __errno, _alloc, and __disable_interrupts foreign procedure conventions

[mflatt]

The __errno convention makes a call for a foreign procedure return two values: the foreign procedure's direct result and the value of errno right after the foreign procedure returns.

On Windows, current-errno-source selects which errno is used for __errno. Also, __get_last_error is available as an alternative to __errno.

Since all backends are affected, I ran at least the foreign.ms test on all architecture+ABI combinations.

[jltaylor-us]

#1009, #550

[mflatt]

The latest revision includes two more commits for two more conventions:

• __alloc modifies a preceding __atomic to indicate that the foreign function doesn't call back to Scheme, but it may call back to the Scheme C library routine to allocate Scheme values. Many functions in the Scheme kernel have this property, such as core bignum routines.
• __disable_interrupts disables interrupts during a call or callable. It's not so useful for a call, since a with-interrupts-disabled wrapper works just as well. It's intended for use with callables, where __disable_interrupts ensures that interrupts are disabled even before reaching the callable body (where trying to call disable-interrupts is probably too late) and after leaving the callable body (where enable-interrupts to restore would be too early).

The commit to add __alloc also adds __atomic to __atomic __alloc to many foreign-procedure forms in the Chez Scheme implementation. I'll post a follow-up with details, but the effect is generally a 0% to 10% improvement for operations like = on bignums, ephemeron-cons, or random. Those annotations could use a close look to make sure they're correct. I did not try to annotate I/O foreign procedures, because they often raise exceptions.

[mflatt]

Here's how I measured the performance effect of __atomic and __atomic __alloc additions to Chez Scheme's implementation, including the "before" and "after" results on my machine (Mac M2).

atomic.txt