Add initial JSON formatter

It works and can format all "plain" Perl data, but has a few known bugs
and limitations that still need to be worked out.

It's about 8x smaller than JSON::XS's encoder and *much* smaller than
Cpanel::JSON::XS, but this is just a first attempt, it'll grow.

c/common.c

@@ -0,0 +1,43 @@
+/* Custom string builder, comparable to functionality provided by SV*
+ * functions, but with less magic and better inlineable. */
+
+typedef struct {
+    size_t len;
+    size_t size;
+    char *buf;
+} fustr;
+
+/* No need to call this, an empty fustr is already usable.
+ * This allows setting a custom initial size. */
+static void fustr_init(fustr *s, size_t prealloc) {
+    s->len = 0;
+    s->size = prealloc;
+    s->buf = safemalloc(prealloc);
+}
+
+static void fustr_grow(fustr *s, size_t add) {
+    if (s->size == 0) s->size = 512;
+    while (s->size < s->len + add)
+        s->size *= 2;
+    s->buf = saferealloc(s->buf, s->size);
+}
+
+#define fustr_reserve(s, n) do {\
+        if (UNLIKELY((s)->size < (s)->len + (n))) fustr_grow(s, n);\
+    } while(0)
+
+#define fustr_write(s, str, n) do {\
+        fustr_reserve(s, n);\
+        memcpy((s)->buf+(s)->len, str, (n));\
+        (s)->len += (n);\
+    } while(0) 
+
+/* Move the string buffer into a new SV; fustr should be considered invalid after this call.
+ * Does not set the UTF8 flag. */
+static SV *fustr_sv(fustr *s) {
+    SV *r = newSV(0);
+    fustr_write(s, "", 1); // trailing nul
+    sv_usepvn_flags(r, s->buf, s->len-1, SV_HAS_TRAILING_NUL);
+    // TODO: SvPV_shrink_to_cur?
+    return r;
+}