mode: reset mode on entry to fn body.

This is an interim fix to address the "Beware!" unsoundness.  I have a
more comprehensive rewrite of mode.rs in the pipeline.

r=pcwalton
tests: Add two test cases, one xfailed
correct expected error msgs in various tests rs=breakage
Add test cases: one xfailed, one not
Add working test for issue 3935 and xfailed test for issue 3888
add two more copies required in word-count rs=breakage
tutorial: Remove memory model section
Add xfailed test for #3973
Use deriving_eq for Task
core: Change XXXs into proper FIXMEs with issue numbers
Update Lindsey's email address
Add core::private::run_in_bare_thread

This begins executing Rust code in a fresh context with no runtime environment