MINOR: compiler: add a TOSTR() macro to turn a value into a string

MINOR: compiler: add a TOSTR() macro to turn a value into a string

Pretty often we have to emit a value (setting, limit etc) in an error
message, and this value is known at compile-time, and just doing this
forces to use a printf format such as "%d". Let's have a simple macro
to turn any other macro or value into a string that can be concatenated
with the rest of the string around. This simplifies error messages
production on the CLI for example.