libstd: Remove mutable fields from fileinput and net_tcp
libstd: Remove mutable fields from flatpipes and io_util
libstd: Remove mutable fields from future and par
libstd: Start warning about mutable fields, now that they only exist in stage0
libcore: Remove mutable fields from rand.
libcore: Remove mutable fields from os and ptr
libcore: Remove mutable fields from hash
libcore: Remove mutable fields from task::spawn
libcore: Remove mutable fields from the task builder API
libcore: Remove mutable fields from pipes
libcore: Remove mutable fields from repr
Remove mutable fields from comm by using casts. Also mark the cast in cell.
libcore: Remove more mutable fields from comm
libstd: Remove some residual mutable fields
librustc: Remove mutable fields from the language.

They're still parsed though, to get through bootstrapping.
auto merge of #6232 : pcwalton/rust/demuting, r=nikomatsakis

They're still parsed for bootstrapping purposes, but the qualifier is immediately dropped on the floor.

r? @nikomatsakis