Define a method for `hash(::Type, ::UInt)` (#49636)
Currently, `hash(::Type, ::UInt)` uses `objectid`, which can have some
odd behavior for types: in particular, subsequent identical type-valued
variable definitions can have `objectid`s which differ from the first
such definition. This has some bizarre downstream effects when e.g.
using types as the values of a `Set` or the keys of a `Dict`. See issue
49620 for examples.
There is an internal `type_hash` C function used for caching types but
isn't exposed to Julia, as Jameson pointed out in the linked issue. This
commit exposes it as `jl_type_hash` which is then used via `ccall` to
define a method `hash(::Type, ::UInt)`. This method then fixes #49620.
Note, however, that this does not affect the differing `objectid`s for
otherwise identical types.
