Previously, "type" functions were reused a lot to manipulate kinds, and other
metadata defined via `define-syntax-category`, but this meant it was impossible
to define separate behavior for some type and kind operations, e.g., type=? and
kind=?. This commit defines a separate api for each `define-syntax-category`
declaration.
Also, every `define-syntax-category` defines a new `define-NAMEd-syntax` form,
which implicitly uses the proper parameters, e.g., `define-kinded-syntax` uses
`kindcheck?`, `current-kind-eval`, and the ':: kind key by default (whereas
before, it was using typecheck?, type-eval, etc).
This commit breaks backwards compatibility. The most likely breakage results
from using a different default key for kinds. It used to be ':, the same as
types, but now the default is '::.
This commit also generalizes the contexts used with `define-NAMEd-syntax` and
`infer`.
- all contexts now accept arbitrary key-values associated with a variable
- all contexts use let* semantics, where a binding is in scope for subsequent
bindings; this means that one environment is sufficient in most scenarioes,
e.g., type and term vars can be mixed (if properly ordered)
- environments allow lone identifiers, which are treated as type variables by
default
