Add Support for Tuples (aka Structs) in the ABI Contract Schema

[mikeseese]

This PR upgrades ajv to v6 which implements draft07 of the JSON Schema. It also adds support for tuples (which is how struct Solidity types are represented) as a parameter type.

You'll see https://github.com/trufflesuite/truffle/pull/2064/files#diff-da060bd5170ce9e90e14ece903806d3eR135 says that the components property has an array of object types. This is supposed to be "items": { "$ref": "#/definitions/Parameter" }, but the https://github.com/bcherny/json-schema-to-typescript dependency we use to generate typings fails due to the recursion (gets a Maximum call stack size exceeded error when building), despite the schema supports this recursion.

The dependency has known about this issue for 2 years now (bcherny/json-schema-to-typescript#76 which depends on another dependency issue APIDevTools/json-schema-ref-parser#37), but a fix has yet to come.

#2065 tracks this issue

[coveralls]

Coverage remained the same at 69.79% when pulling c209b53 on bug/contract-schema-tuples into cadba21 on develop.

[gnidan]

This is great! Thanks for reporting those findings 💯