basic/macros: add STRLEN() to get length of string literal as constant expression

basic/macros: add STRLEN() to get length of string literal as constant expression

While the compiler likely optimizes strlen(x) for string literals,
it is not a constant expression.

Hence,

  char buffer[strlen("OPTION_000") + 1];

declares a variable-length array. STRLEN() can be used instead
when a constant espression is needed.

It's not entirely identical to strlen(), as STRLEN("a\0") counts 2.
Also, it only works with string literals and the macro enforces
that the argument is a literal.