cassette method overloads not respected

[jrevels]

Originally thought this was a straight up world age issue a la #6, but getting rid of the type parameter in the prehook definition "fixes" it...

@JeffBezanson
@vtjnash

julia> using Cassette

julia> Cassette.@context Ctx

julia> ctx = Ctx()
Ctx{Cassette.Unused,Cassette.Unused,Nothing}(Cassette.Unused(), Cassette.Unused(), nothing)

julia> Cassette.overdub_recurse(ctx, sin, 1)
0.8414709848078965

# Doing the following instead makes this work instead of break:
# 
#   Cassette.prehook(::Ctx, f::Any, args...) = println("1")
#
julia> Cassette.prehook(::C, f::Any, args...) where {C<:Ctx} = println("1")

julia> Cassette.overdub_recurse(ctx, sin, 1)
0.8414709848078965

julia> methods(Cassette.prehook)
# 2 methods for generic function "prehook":
[1] prehook(::C, f, args...) where C<:Ctx in Main at REPL[6]:1
[2] prehook(::Cassette.AbstractContext, ...) in Cassette at /Users/jarrettrevels/.julia/v0.7/Cassette/src/overdub.jl:5

julia> @which Cassette.prehook(ctx, sin, 1)
prehook(::C, f, args...) where C<:Ctx in Main at REPL[6]:1

# reflection gives us the right method, but it's still not getting called
julia> Cassette.prehook(ctx, sin, 1)

[jrevels]

Here's a non-Cassette version of a similar case that does not exhibit the same issue: https://gist.github.com/jrevels/1a41f9f1682acf324fc0e4b002bc6a77

...so a non-Cassette MWE of this lies somewhere "between" Cassette and the above.