Define schema for representing data types

[gnidan]

(Pardon the short description and lack of commit sanity, but I'm opening this PR in hopes Fleek will deploy a preview build. [didn't work... stealing the main Fleek URL])

This PR proposes to define the ethdebug/format/type JSON schema for representing known and unknown data types in a machine-readable way (so that a compiler can tell a debugger about types).

This takes the organization approach to do the following:

• Define a base schema that doesn't know about uints or strings, etc. It defines the syntactic form of elementary types vs. complex types (types that compose other types). [I imagine we'll add algebraic types to this distinction soon]
• Define a canonical type schema that imposes restrictions on known kinds of types, like to assert that uint types must be specified with a certain number of bits.

Current status

• This PR includes an overview of the ethdebug/format/type schema and explanation of the approach described above. See page preview online.
• This PR includes an initial draft of ethdebug/format/type/base, along with explanation of key concepts and some examples. See page preview online.
• This PR includes placeholder files for the full schema / spec page about it. Don't bother seeing page preview online.

[gnidan]

Closing this in favor of #48