Also serialize dt->instance. Fixes #5129

dt->instance is one of the jl_value_t*'s that can occur purely in code
and not anywhere else in the AST. All other instances of where this
happends are captured by li->roots, since they need GC roots that way,
but this instance is rooted by the owning type.
Merge pull request #5149 from JuliaLang/kf/5129

Also serialize dt->instance. Fixes #5129