FU: Support multipart file uploads + some doc fixes

API is not super convenient and implementation is lousy, but uploading
files is not a super common operation so that should be fine.

At least it supports large files with only a single in-memory copy.

t/multipart.t

@@ -0,0 +1,47 @@
+use v5.36;
+use Test::More;
+use FU::MultipartFormData;
+
+# Example based on https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST
+my $t = <<'_' =~ s/\n/\r\n/rg;
+--delimiter12345
+Content-Disposition: form-data; name="field1"
+content-type: hello; charset=x
+
+value1
+--delimiter12345
+Content-Type: text
+Content-Disposition: form-data; filename="example.txt"; name=field2
+
+value2
+--delimiter12345--
+_
+
+
+my $l = FU::MultipartFormData->parse('multipart/form-data;boundary="delimiter12345"', $t);
+is scalar @$l, 2;
+
+my $v = $l->[0];
+is $v->name, 'field1';
+is $v->filename, undef;
+is $v->mime, 'hello';
+is $v->charset, 'x';
+is $v->length, 6;
+is $v->data, 'value1';
+
+is $v->substr(4), 'e1';
+is $v->substr(1, 2), 'al';
+is $v->substr(-2, 1), 'e';
+is $v->substr(-2, 5), 'e1';
+is $v->substr(-100, 2), 'va';
+is $v->substr(1, -3), 'al';
+
+$v = $l->[1];
+is $v->name, 'field2';
+is $v->filename, 'example.txt';
+is $v->mime, 'text';
+is $v->charset, undef;
+is $v->length, 6;
+is $v->data, 'value2';
+
+done_testing;