rust-mode.el: Tweak syntax table

"_" should keep the default syntax class (symbol, not word). This
allows, e.g., `forward-word' to behave in the familiar way, jumping to
underscores within a function or variable name.
Don't call drop in tcpstream docs

This suggests that you must call it, which is normally not what you want to do.
auto merge of #18818 : steveklabnik/rust/tcp_doc, r=alexcrichton

This suggests that you must call it, which is normally not what you want to do.

See here: http://www.reddit.com/r/rust/comments/2lpw9k/rust_irc_example_looking_for_feedback/clxnxxc?context=4
auto merge of #19021 : roysc/rust/emacs-pr, r=brson

"_" should keep the default syntax class (symbol, not word). This
allows, e.g., `forward-word' to behave in the familiar way, jumping to
underscores within a function or variable name.
std: Leak all statically allocated TLS keys

It turns out that rustrt::at_exit() doesn't actually occur after all pthread
threads have exited (nor does atexit()), so there's not actually a known point
at which we can deallocate these keys. It's not super critical that we do so,
however, because we're about to exit anyway!

Closes #19280
auto merge of #19285 : alexcrichton/rust/issue-19280, r=aturon

It turns out that rustrt::at_exit() doesn't actually occur after all pthread
threads have exited (nor does atexit()), so there's not actually a known point
at which we can deallocate these keys. It's not super critical that we do so,
however, because we're about to exit anyway!

Closes #19280