New schema format

[aumetra]

Closes #1502

This is a PR adding a new schema MVP that intends to encode Rust types a little better than JSON schema does.

The biggest differences are:

• Full support for all kinds of enums Rust supports (tuple, named, unit) and enums without cases (so non-constructable types such as core::convert::Infallible)
• Full support for all kinds of structs Rust supports (named, tuple, unit)
• Full support for tuples
• Dedicated HexBinary, Timestamp, Optional, Checksum, Binary, and Address types
• Slightly more compact representation through using numeric indices vs. path names
• Support for arbitrary-size integers by encoding their precision and whether they are signed
• Support for arbitrary-size decimals by encoding their precision and wehther they are signed

[aumetra]

God those warnings I added.. they're really "we have compiler warnings at home" material