Removed mut-fields from Mut, removed borrow_const from Mut
Removed all uses of Mut from workcache, replaced with @mut
Removed libcore/mutable.rs, Mut<T> is now dead.
auto merge of #5678 : Thiez/rust/no-Mut, r=thestinger

This pull request completely removes Mut<T> and the associated file (libcore/mutable.rs). Some minor changes were made to workcache (libstd/workcache.rs) as it was using Mut.

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