parsing size limits

[jvatic]

While metadata tends to be fairly small in size (especially when running on a computer with resources to spare), the input file should be treated as untrusted and therefore it's a bad idea to assume this.

I'd like to introduce configurable size limits when parsing (exceeding any of them is a parse error) for

• any atom (perhaps configurable per atom type as well as generally?)
• ilst items (perhaps configurable per item type as well as generally?)
• overall metadata (limit both size and number of atoms)
• mdat

The defaults for these can be something no reasonable file should exceed.