Add curl MLton FFI bindings

Getting the data at a URL can be a useful thing to do. The best way to
do it is probably with libcurl bindings. This unfortunately requires a
small C wrapper to do some bookkeeping (curl only wants to write via a
function pointer or a FILE*) but it's still pretty braindead.

curl/README

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+This attempts to be a very simple interface to libcurl. Pass in your URL, get the data at that URL back.
+
+Because MLton's build system is half-assed, in order to use this lib you must:
+ - add "-link-opt -lcurl" to your mlton command line parameters
+ - add ".../stilts/curl/curl_supereasy.c" after your mlb file on the mlton command line

curl/curl.mlb

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+ann
+	"allowFFI true"
+in
+	$(SML_LIB)/basis/basis.mlb
+	$(SML_LIB)/basis/mlton.mlb
+	$(SML_LIB)/basis/c-types.mlb
+	curl.sig
+	curl.sml
+end

curl/curl.sig

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+signature CURL = sig
+	val curl : string -> string
+end

curl/curl.sml

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+structure Curl :> CURL = struct
+
+	type ptr = MLton.Pointer.t
+
+	fun read_bytes (p, n) =
+		Word8Vector.tabulate (n, fn i => MLton.Pointer.getWord8 (p, i))
+
+	fun curl url =
+	let
+		val p =
+			(_import "curl_supereasy" : string -> ptr;)
+			(url)
+
+		val len =
+			(_import "curl_supereasy_len" : ptr -> int;)
+			(p)
+
+		val data =
+			(_import "curl_supereasy_data" : ptr -> ptr;)
+			(p)
+
+		val ret = Byte.bytesToString (read_bytes (data, len))
+
+		val _ =
+			(_import "curl_supereasy_cleanup" : ptr -> unit;)
+			(p)
+	in
+		ret
+	end
+
+end

curl/curl_supereasy.c

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+#include <stdlib.h>
+#include <string.h>
+#include <curl/curl.h>
+
+typedef struct
+{
+	char *data;
+	size_t len;
+}
+curl_supereasy_data_t;
+
+size_t curl_supereasy_callback(
+	void *buffer,
+	size_t size,
+	size_t nmemb,
+	curl_supereasy_data_t *datat)
+{
+	size_t old_len = datat->len;
+	size_t additional_len = size*nmemb;
+
+	datat->data = realloc(datat->data, old_len + additional_len);
+	datat->len = old_len + additional_len;
+
+	memcpy(datat->data + old_len, buffer, additional_len);
+
+	return additional_len;
+}
+
+curl_supereasy_data_t* curl_supereasy(char *url)
+{
+	curl_supereasy_data_t *datat = malloc(sizeof(curl_supereasy_data_t));
+	datat->data = NULL;
+	datat->len = 0;
+
+	CURL *curl = curl_easy_init();
+	curl_easy_setopt(curl, CURLOPT_URL, url);
+	curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, curl_supereasy_callback);
+	curl_easy_setopt(curl, CURLOPT_WRITEDATA, datat);
+	curl_easy_perform(curl);
+	curl_easy_cleanup(curl);
+
+	return datat;
+}
+
+char* curl_supereasy_data(curl_supereasy_data_t *datat)
+{
+	return datat->data;
+}
+
+size_t curl_supereasy_len(curl_supereasy_data_t *datat)
+{
+	return datat->len;
+}
+
+void curl_supereasy_cleanup(curl_supereasy_data_t *datat)
+{
+	free(datat->data);
+	free(datat);
+}