Everywhere: Replace single-char StringView op. arguments with chars

This prevents us from needing a sv suffix, and potentially reduces the
need to run generic code for a single character (as contains,
starts_with, ends_with etc. for a char will be just a length and
equality check).

No functional changes.

Tests/Kernel/fuzz-syscalls.cpp

@@ -100,7 +100,7 @@ static void do_weird_call(size_t attempt, int syscall_fn, size_t arg1, size_t ar
             builder.append(", "sv);
         builder.appendff("{:p}", fake_params[i]);
     }
-    builder.append("]");
+    builder.append(']');
     dbgln("{}", builder.build());
 
     // Actually do the syscall ('fake_params' is passed indirectly, if any of arg1, arg2, or arg3 point to it.