Make P6 "int" always match C "int" (or provide an alternative)

[p6rt]

Migrated from rt.perl.org#132864 (status was 'new')

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[p6rt]

From @jimav

Hello,

Please see Raku/doc#1771 for motivation.

In brief​: The P6 "native" type int is currently not guaranteed to be the
same as an "int" in C, which prevents portably using "int" with
NativeCall to interface to C code which has "int" arguments.

My suggestion​:

Officially guarantee that P6 "int" will always match a C "int" on the
current platform.  Document that portable code should assume a range of
-32768..32767 (16 bits), which is the minimum required by C99.
This change will allow "int" to be used portably with NativeCall and
DWIM on all platforms.

Alternatively​:

Define another "native" type which _is_ guaranteed to be the same as a C
"int", and document it's use with NativeCall.
If this solution is taken, then make the compiler emit an error if "int"
is used in a CStruct or similar constructs, to prevent people from
thinking they are writing portable code.

---

Relatedly, there should be a native "unsigned" which is guaranteed to
match a C "unsigned", for use with NativeCall.

-Jim