forbid cast as bool

This is currently unsound since `bool` is represented as `i8`. It will
become sound when `bool` is stored as `i8` but always used as `i1`.

However, the current behaviour will always be identical to `x & 1 != 0`,
so there's no need for it. It's also surprising, since `x != 0` is the
expected behaviour.

Closes #7311
std::str: Deny surrogates in is_utf8

Reject codepoints \uD800 to \uDFFF which are the surrogates
(reserved/unused codepoints that are invalid to encode into UTF-8)

The surrogates is the only hole of invalid codepoints in the range from
\u0 to \u10FFFF.
auto merge of #8980 : thestinger/rust/bool, r=thestinger

This is currently unsound since `bool` is represented as `i8`. It will
become sound when `bool` is stored as `i8` but always used as `i1`.

However, the current behaviour will always be identical to `x & 1 != 0`,
so there's no need for it. It's also surprising, since `x != 0` is the
expected behaviour.

Closes #7311

d0a1176 r=huonw
e4a76e6 r=thestinger