C: Optimize hb_string() to compute length at compile time (#1318)

This pull request converts the `hb_string()` function to a macro and
renames the existing `hb_string` function to `hb_string_from_c_string`.

We do this in order to optimize `hb_string()` calls with static string
literals so that they have no runtime-overhead for converting a C-String
to a `hb_string_T`.

This applies for calls like:
```c
hb_string("<%")
hb_string("%>")
```
For dynamic strings it will fallback to `hb_string_from_c_string()`:
```c
hb_string(variable)
hb_string(message)
```

Author: marcoroth

src/include/util/hb_string.h

@@ -15,7 +15,13 @@ typedef struct HB_STRING_STRUCT {
 #define HB_STRING_EMPTY ((hb_string_T){ .data = "", .length = 0 })
 #define HB_STRING_NULL  ((hb_string_T){ .data = NULL, .length = 0 })
 
-hb_string_T hb_string(const char* null_terminated_c_string);
+hb_string_T hb_string_from_c_string(const char* null_terminated_c_string);
+
+#define hb_string(string) \
+  (__builtin_constant_p(string) \
+    ? ((hb_string_T){ .data = (char*)(string), .length = (uint32_t)__builtin_strlen(string) }) \
+    : hb_string_from_c_string(string))
+
 hb_string_T hb_string_slice(hb_string_T string, uint32_t offset);
 
 bool hb_string_equals(hb_string_T a, hb_string_T b);

src/util/hb_string.c

@@ -6,7 +6,7 @@
 #include <string.h>
 #include <strings.h>
 
-hb_string_T hb_string(const char* null_terminated_c_string) {
+hb_string_T hb_string_from_c_string(const char* null_terminated_c_string) {
   if (null_terminated_c_string == NULL) { return HB_STRING_NULL; }
 
   hb_string_T string;