Try to sniff the mime type based on the body content

Synchronous body reader now exposes a peek() function to get the first few bytes
from the response body. This will be no less than 100 bytes (assuming the body
is that big), but could be more. Streaming API, via res.next() continues to work
as-is even if peek() is called.

Introduce Mime.sniff() that detects a few common types - the ones that we care
about right now - from the body content.

src/testing.zig

@@ -24,6 +24,17 @@ pub const expectError = std.testing.expectError;
 pub const expectString = std.testing.expectEqualStrings;
 pub const expectEqualSlices = std.testing.expectEqualSlices;
 
+// sometimes it's super useful to have an arena you don't really care about
+// in a test. Like, you need a mutable string, so you just want to dupe a
+// string literal. It has nothing to do with the code under test, it's just
+// infrastructure for the test itself.
+pub var arena_instance = std.heap.ArenaAllocator.init(std.heap.c_allocator);
+pub const arena_allocator = arena_instance.allocator();
+
+pub fn reset() void {
+    _ = arena_instance.reset(.{ .retain_capacity = {} });
+}
+
 const App = @import("app.zig").App;
 const parser = @import("browser/netsurf.zig");