etl/span.h: prevent compound statements in constexpr methods for C++11 (#562)
etl/span.h: prevent compound statements in constexpr methods for C++11 (#562)

C++11 doesn't support compound statements in constexpr functions
and as a result, `etl/span.h` generates a heap of warnings with
gcc's `-Wpedantic` in C++11 mode. This changes the relevant method
bodies to contain just a single return statement.
ETLCPP/etl,ETLCPP/etl,ETLCPP/etl,ETLCPP/etl