Removed drop from the keyword list in the docs
auto merge of #7582 : mitsuhiko/rust/doc-fixes, r=huonw

Drop is no longer a keyword, removed it from the intro docs.
Fix #3225: Check for old Rust libraries when building and installing.

When building Rust libraries (e.g. librustc, libstd, etc), checks for
and verbosely removes previous build products before invoking rustc.
(Also, when Make variable VERBOSE is defined, it will list all of the
libraries matching the object library's glob after the rustc
invocation has completed.)

When installing Rust libraries, checks for previous libraries in
target install directory, but does not remove them.

The thinking behind these two different modes of operation is that the
installation target, unlike the build tree, is not under the control
of this infrastructure and it is not up to this Makefile to decide if
the previous libraries should be removed.