Update janet_pcall interface

The programmer can now not only get the used fiber, but
provide a fiber to reuse if many calls are made in succession.

src/core/io.c

@@ -198,6 +198,9 @@ static Janet cfun_io_fread(int32_t argc, Janet *argv) {
                 if (fsize < 0) {
                     janet_panicf("could not get file size of %v", argv[0]);
                 }
+                if (fsize > (INT32_MAX)) {
+                    janet_panic("file to large to read into buffer");
+                }
                 fseek(iof->file, 0, SEEK_SET);
                 read_chunk(iof, buffer, (int32_t) fsize);
             }

src/core/peg.c

@@ -1057,7 +1057,6 @@ static Janet cfun_peg_match(int32_t argc, Janet *argv) {
     s.captures = janet_array(0);
     s.scratch = janet_buffer(10);
     s.tags = janet_buffer(10);
-
     s.constants = peg->constants;
     s.bytecode = peg->bytecode;
     const uint8_t *result = peg_rule(&s, s.bytecode, bytes.bytes + start);

src/core/vm.c

@@ -852,7 +852,12 @@ JanetSignal janet_pcall(
     const Janet *argv,
     Janet *out,
     JanetFiber **f) {
-    JanetFiber *fiber = janet_fiber(fun, 64, argc, argv);
+    JanetFiber *fiber;
+    if (f && *f) {
+        fiber = janet_fiber_reset(*f, fun, argc, argv);
+    } else {
+        fiber = janet_fiber(fun, 64, argc, argv);
+    }
     if (f) *f = fiber;
     if (!fiber) {
         *out = janet_cstringv("arity mismatch");