lang/polyml: 5.5.2 -> 5.6

Changes:

• New foreign-function interface (Foreign structure).
• Reimplemented debugger so that code with debugging enabled is much faster.
• Improved support for Poly/ML on native Windows.
• Support for MIPS and 64-bit ARM processors through the byte-code interpreter.
• Various bug fixes.

PR: 208783, 199566
Submitted by: Phil Eaton <philneaton95@gmail.com>, brooks
Approved by: Timothy Beyer <beyert@cs.ucr.edu> (maintainer)