add int/to-number: converts s64 and u64 to numbers

(int/to-number value) converts an s64 or u64 to a number.
It restricts the value to the int32 range,
so that `int32?` will always suceeded when called on the result.

src/core/inttypes.c

@@ -207,6 +207,38 @@ JANET_CORE_FN(cfun_it_u64_new,
     return janet_wrap_u64(janet_unwrap_u64(argv[0]));
 }
 
+JANET_CORE_FN(cfun_to_number,
+              "(int/to-number value)",
+              "Convert an int/u64 or int/s64 to a number. Fails if the number is out of range for an int32.") {
+    janet_fixarity(argc, 1);
+    if (janet_type(argv[0]) == JANET_ABSTRACT) {
+        void* abst = janet_unwrap_abstract(argv[0]);
+
+        if (janet_abstract_type(abst) == &janet_s64_type) {
+            int64_t value = *((int64_t*)abst);
+            if (value > INT32_MAX) {
+                janet_panicf("cannot convert %q to a number, exceededs int32 max", argv[0]);
+            }
+            if (value < INT32_MIN) {
+                janet_panicf("cannot convert %q to a number, exceededs int32 min", argv[0]);
+            }
+
+            return janet_wrap_integer(value);
+        }
+
+        if (janet_abstract_type(abst) == &janet_u64_type) {
+            uint64_t value = *((uint64_t*)abst);
+            if (value > INT32_MAX) {
+                janet_panicf("cannot convert %q to a number, exceededs int32 max", argv[0]);
+            }
+
+            return janet_wrap_integer(value);
+        }
+    }
+
+    janet_panicf("expected int/u64 or int/s64, got %q", argv[0]);
+}
+
 /*
  * Code to support polymorphic comparison.
  * int/u64 and int/s64 support a "compare" method that allows
@@ -514,6 +546,7 @@ void janet_lib_inttypes(JanetTable *env) {
     JanetRegExt it_cfuns[] = {
         JANET_CORE_REG("int/s64", cfun_it_s64_new),
         JANET_CORE_REG("int/u64", cfun_it_u64_new),
+        JANET_CORE_REG("int/to-number", cfun_to_number),
         JANET_REG_END
     };
     janet_core_cfuns_ext(env, NULL, it_cfuns);

test/suite0006.janet

@@ -39,6 +39,20 @@
    (def c (u64 "32rvv_vv_vv_vv"))
    (def d (u64 "123456789"))))
 
+# Conversion back to an int32
+(assert (= (int/to-number (u64 0xFaFa)) 0xFaFa))
+(assert (= (int/to-number (i64 0xFaFa)) 0xFaFa))
+
+(assert-error
+  "int64 out of bounds for int32"
+  (do
+    (int/to-number (u64 "0x7fff_ffff_ffff_ffff"))
+    (int/to-number (i64 "-0x7fff_ffff_ffff_ffff"))))
+
+(assert-error
+  "int/to-number fails on non-abstract types"
+  (int/to-number 1))
+
 (assert-no-error
  "create some int64 bigints"
  (do