convert Sb macro to a proper function

...this way we can assert that the provided buffer is actually
null-terminated.
`Sb()` -> `Str_from_buf()`.

compiler/pre.h

@@ -86,8 +86,6 @@ typedef Slice(u8) Str;
 
 /* Creates a `Str` from a string literal */
 #define Sl(s) ((Str){ (u8 *)s, (isize)lengthof(s) })
-/* Creates a `Str` from a buffer of size `len` */
-#define Sb(s, len) ((Str){ (u8 *)s, (isize)len })
 /* Creates a `Str` from a C string. */
 #define Str_from_c(s) ((Str){ (u8 *)s, (isize)(s != nil ? strlen(s) : 0) })
 #define Str_empty(s) ((s).len == 0)
@@ -96,6 +94,16 @@ typedef Slice(u8) Str;
 int
 vsnprintf(char *, unsigned long, const char *, va_list);
 
+/* Creates a `Str` from a buffer of size `len` */
+static inline Str
+Str_from_buf(u8 *buf, isize len)
+{
+	/* If this triggers an OOB, its because the code that created the string was
+	 * broken anyways. ASan will catch it. */
+	Assert(buf[len] == '\0');
+	return (Str){ buf, len };
+}
+
 /* "Converts" a `Str` into a C string. Since `Str` are meant to be
  * null-terminated already, no conversion is made, but ensures that the
  * null terminator is present. */