Swap out unboxed_closures feature gate for min_specialization
Swap out unboxed_closures feature gate for min_specialization

For some reason unboxed_closures supresses the feature gate for
min_specialization when implementing TrustedStep. min_specialization is
the true feature that is used.

aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust