Record where thunks actually created

[oxinabox]

Is this enough to close #57 ?

Should I test this? Maybe can throw error, then access the backtrack as a string,
and then make sure the right file name occurs?

Before

julia> demo_pb() = @thunk(error("no"))
demo_pb (generic function with 1 method)

julia> extern(demo_pb())
ERROR: no
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] (::var"#15#16")() at /Users/oxinabox/JuliaEnvs/ChainRulesWorld/ChainRulesCore.jl/src/differentials.jl:216
 [3] (::Thunk{var"#15#16"})() at /Users/oxinabox/JuliaEnvs/ChainRulesWorld/ChainRulesCore.jl/src/differentials.jl:223
 [4] extern(::Thunk{var"#15#16"}) at /Users/oxinabox/JuliaEnvs/ChainRulesWorld/ChainRulesCore.jl/src/differentials.jl:224
 [5] top-level scope at REPL[12]:1

After

julia> extern(demo_pb())
ERROR: no
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] (::var"#13#14")() at ./REPL[9]:1
 [3] (::Thunk{var"#13#14"})() at /Users/oxinabox/JuliaEnvs/ChainRulesWorld/ChainRulesCore.jl/src/differentials.jl:232
 [4] extern(::Thunk{var"#13#14"}) at /Users/oxinabox/JuliaEnvs/ChainRulesWorld/ChainRulesCore.jl/src/differentials.jl:233
 [5] top-level scope at REPL[10]:1

Note that in the real use case when not testing it in the REPL
will whow a filename and line number rather than
[2] (::var"#13#14")() at ./REPL[9]:1

[nickrobinson251]

Made you a label 🏷 😀

[nickrobinson251]

I really like this. Just needs tests :)