FU::Validate: Add module + integrate with FU

Copied from TUWF::Validate with a few small changes. I have a few more
features planned, but let's see how this goes first.

It's been an incredibly useful module in the past, I'm not sure right
now if I had ideas for potential improvements at some point, will need
to check notes.

FU.pm

@@ -604,14 +604,24 @@ sub header($, $h) { $FU::REQ->{hdr}{ lc $h } }
 sub headers { $FU::REQ->{hdr} }
 sub ip { $FU::REQ->{ip} }
 
+sub _getfield($data, @a) {
+    return $data->{$a[0]} if @a == 1 && !ref $a[0];
+    require FU::Validate;
+    my $schema = FU::Validate->compile(@a > 1 ? { keys => {@a} } : $a[0]);
+    my $res = $schema->validate($data);
+    fu->error(400, "Input validation failed") if !$res; # TODO: More detailed error message
+    return @a == 2 ? $res->data->{$a[0]} : $res->data;
+}
+
 sub query {
-    return $FU::REQ->{qs} if @_ == 1;
+    shift;
+    return $FU::REQ->{qs} if !@_;
     $FU::REQ->{qs_parsed} ||= eval { FU::Util::query_decode($FU::REQ->{qs}) } || fu->error(400, $@);
-    # TODO: Also accept schema validation thing.
-    $FU::REQ->{qs_parsed}{$_[1]};
+    _getfield $FU::REQ->{qs_parsed}, @_;
 }
 
 sub formdata {
+    shift;
     $FU::REQ->{formdata} ||= eval {
         # TODO: Support multipart encoding
         confess "Invalid content type for form data"
@@ -619,7 +629,7 @@ sub formdata {
         FU::Util::query_decode($FU::REQ->{data});
     } || fu->error(400, $@);
     # TODO: Accept schema validation thing.
-    $FU::REQ->{formdata}{$_[1]};
+    _getfield $FU::REQ->{formdata}, @_;
 }
 
 
@@ -849,6 +859,8 @@ standalone and can be used independently of the framework:
 
 =item * L<FU::SQL> - Small and safe query builder.
 
+=item * L<FU::Validate> - Input validation through a schema.
+
 =item * L<FU::XMLWriter> - Dynamic XML generation, easy and fast.
 
 =item * L<FU::Log> - Global logger.
@@ -1142,11 +1154,30 @@ C<https://example.com/some/path?query> this returns C<query>.
 =item fu->query($name)
 
 Parses the raw query string with C<query_decode> in L<FU::Util> and returns the
-value with the given $name.
+value with the given $name. Beware: multiple values are returned as an array.
+Prefer to use the C<$schema>-based validation methods below to reliably handle
+all sorts of query strings.
+
+=item fu->query($name => $schema)
+
+Parse, validate and return the query parameter identified by C<$name> with the
+given L<FU::Validate> schema. Calls C<< fu->error(400) >> with a useful error
+message if validation fails.
 
 =item fu->query($schema)
 
-I<TODO>
+=item fu->query($name1 => $schema1, $name2 => $schema2, ..)
+
+Parse, validate and return multiple query parameters.
+
+  state $schema = FU::Validate->compile({
+      keys => { a => {anybool => 1}, b => {} }
+  });
+  my $data = fu->query($schema);
+  # $data = { a => .., b => .. }
+
+  # Or, more concisely:
+  my $data = fu->query(a => {anybool => 1}, b => {});
 
 =item fu->formdata($name)