Release cpp11 0.5.4

Unable to install the latest version of cpp11

`as_sexp_strings()` could use a single `unwind_protect()` around the loop

Making things a little bit faster

cpp_source fails to compile indented void-returning function

Values added to a vector with `push_back` cannot be used immediately

Global symbol visibility / deriving from a C++ class defined in a different shared library

New CRAN warning: identifier preceded by whitespace in a literal operator declaration

Another rchk issue to look into

Cannot turn off `checking of pragma(s) suppressing important diagnostics` when including external headers