[compiler, runtime] add FFI callbacks support

[quasilyte]

• Improved PHP FFI nullptr handling (+ added tests #456)
• Added FFI callbacks support (#450 #425)
• Added unmanaged FFI memory allocation
• Fixed double-vs-float token issue in FFI (#584)
• Added @kphp-ffi-signalsafe annotation for C headers (cdef #586)
• Fixed imprecise C parser error locations (#446)
• Added ffi_cast_addr2ptr and ffi_cast_ptr2addr functions

Unmanaged memory is handles like this:

• When unmanaged chunk is allocated, a refcnt is 2 instead of 1
• So, this memory is never deallocated via that object
• When free is used on such object, refcnt is decreased
• If that was the last reference, the memory will be deallocated
  • Otherwise it will be deallocated when the last reference dies
  • It's UB to call free on the same memory chunk more than once

The memory can be deallocated via the non-owning pointer like CDataPtr; it's UB to call free on the owning object after that. There should be exactly 1 free call per every allocated memory chunk. The FFI documentation describes that and shares a few common patterns.

FFI callback functions have restrictions placed on them:

• Can't throw from it
• Can't have captured state (no instance methods, no closures)
• Can't be resumable

Can use stateless lambdas, freestanding functions, and static methods as FFI callbacks.

Fixes #450 Fixes #425 Fixes #456 Fixes #585 Fixes #584 Fixes #586 Fixes #451 Fixes #446