avoid specializing `convert` for `Any` result types

[stev47]

Specializing convert(Result{Any, E}, x) for every type of x can incur substantial compilation overhead (not only on convert but also on Some). This change avoids specializing in case of explicit Any result type.

This is analogous to Julia Base where we have

convert(::Type{Any}, Core.@nospecialize x) = x

to avoid excessive specialization.