Convert constant macros to constexpr expressions

Also convert some `const[expr] static xxx` to `const[expr] xxx` where it makes
sense to let the compiler deduce on its own whether or not to allocate storage
for a constant variable rather than imposing our view that it should have STATIC
storage set aside for it.

A few call sites were not making use of the `XXX_LEN` definitions and were
calling `strlen(XXX)` - these have been updated to use `const_strlen(XXX)`
instead.

I'm not sure if any toolchains will have raise any issues with these changes...
CI will tell!

src/input_common.cpp

@@ -30,7 +30,7 @@
 /// Time in milliseconds to wait for another byte to be available for reading
 /// after \x1B is read before assuming that escape key was pressed, and not an
 /// escape sequence.
-#define WAIT_ON_ESCAPE_DEFAULT 30
+constexpr int WAIT_ON_ESCAPE_DEFAULT = 30;
 static int wait_on_escape_ms = WAIT_ON_ESCAPE_DEFAULT;
 
 input_event_queue_t::input_event_queue_t(int in) : in_(in) {}