parser: implement streaming parser

With my pathological 50MiB 10_000 line nested list test, this is
definitely slower than the one shot parser, but it has peak memory
usage of 5MiB compared to the 120MiB of the one-shot parsing. Not bad.
Obviously this result is largely dependent on the fact that this
particular benchmark is 99% whitespace, which does not get copied into
the resulting document. A (significantly) smaller improvement will be
observed in files that are mostly data with little indentation or
empty lines.

But a win is a win.

examples/stream.zig

@@ -0,0 +1,30 @@
+const std = @import("std");
+
+const nice = @import("nice");
+
+pub fn main() !void {
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+
+    const args = try std.process.argsAlloc(allocator);
+    defer std.process.argsFree(allocator, args);
+    if (args.len < 2) return;
+
+    const document: nice.Document = doc: {
+        const file = try std.fs.cwd().openFile(args[1], .{});
+        defer file.close();
+        var parser = try nice.StreamParser.init(allocator, .{});
+        defer parser.deinit();
+        while (true) {
+            var buf = [_]u8{0} ** 1024;
+            const len = try file.read(&buf);
+            if (len == 0) break;
+            try parser.feed(buf[0..len]);
+        }
+        break :doc try parser.finish();
+    };
+    defer document.deinit();
+
+    document.printDebug();
+}