Drop struct fields if the user destructor fails

This commit changes drop glue generated for structs to use the invoke LLVM
instruction instead of call. What this means is that if the user destructor
triggers an unwinding, then the fields of the struct will still ge dropped.

This is not an attempt to support failing while failing, as that's mostly a
problem of runtime support. This is more of an issue of soundness in making sure
that destructors are appropriately run. The test included fails before this
commit, and only has one call to fail!(), yet it doesn't destroy its struct
fields.
auto merge of #10219 : alexcrichton/rust/drop-invoke, r=pcwalton

This commit changes drop glue generated for structs to use the invoke LLVM
instruction instead of call. What this means is that if the user destructor
triggers an unwinding, then the fields of the struct will still ge dropped.

This is not an attempt to support failing while failing, as that's mostly a
problem of runtime support. This is more of an issue of soundness in making sure
that destructors are appropriately run. The test included fails before this
commit, and only has one call to fail!(), yet it doesn't destroy its struct
fields.
docs: Replace std::iterator with std::iter.
auto merge of #10248 : nibrahim/rust/docformatting, r=pcwalton

The code block shows up inline without proper formatting without this
newline.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
auto merge of #10252 : huonw/rust/docs, r=alexcrichton
simplify memcpy/memmove/memset intrinsics

This moves the per-architecture difference into the compiler.
auto merge of #10251 : thestinger/rust/ptr, r=alexcritchton

This moves the per-architecture difference into the compiler.