[unittest] Make "msg" argument optional for {ASSERT,EXPECT}_STR_{EQ,NE}()

This makes ASSERT/EXPECT_STR_EQ() consistent with the numeric
comparison checks such as EXPECT_EQ(), where the "msg" argument is
already optional.

Simplify some tests by using EXPECT_STR_EQ() directly instead of
defining an EXPECT_CSTR_EQ() helper.

Change-Id: Ia6e697a7492d16b86efe92a4e51c0ecc82749ed2

[unittest] Remove 'length' argument from EXPECT_STR_EQ()/ASSERT_STR_EQ()

The 'length' argument is redundant now that these two functions always
compare the whole strings.

I updated the call sites using an automated tool.

I've removed the now-unneeded EXPECT_STRING_EQ() wrapper in
syslog_socket_tests.cpp.

Change-Id: I80fff5c5ccdc44337125bc2b2d3386fc6606f1dc

[fbl][string] Add assignment operators for interoperability.

This matches the newly added conversion constructors to make it easier
to interoperate with other string-like objects.

Change-Id: I02746d195411a3194f05adcf764989ff7a64170c

[fbl][string] Improve interoperability with std::strings.

Added template constructors to fbl::StringPiece and fbl::String which
accept a variety of string-like objects. This is intended to reduce
the pain of having multiple string representations floating around.

Use the new trait for the tracing API.

Change-Id: Id0c73d386f1286e1e91e94af295a311b80db311b

[zircon][mxtl->fbl] Rename mxtl to fbl

Change-Id: Ie21b6498e1bfb0a7fa0315e40b9e5c3ee78646be