Create a Julia implementation of the C task struct

[yebai]

It might be possible to create a Julia version of the task struct, based on the memory layout information in #89, then we can further simplify the getfield/setfield functions for manipulating the task fields. It might require creating some new bit types (or primitive types) to represent arbitrarily sized memory blocks without clear Julia equivalent type.

[giordano]

creating a Julia struct which mimics the layout of jl_task_t should help with #85. As annoying as maintaining such a (likely version-dependent) struct is, not having to build an external library just to call into libjulia looks a big win to me.

[fingolfin]

Note that ccalls to functions like jl_getfield_ptr or jl_setfield_int32_t etc. could be replaced by calls to unsafe_load resp. unsafe_store!

[yebai]

Closed in favour of #93