Use a smaller size for the lexer buffer

Large sizes are more efficient when it's actually buffered,
but most of the time `mmap` is used instead, and the extra size
just slows down allocation of lexer states.

include/asm/lexer.hpp

@@ -7,14 +7,15 @@
 #include <memory>
 #include <optional>
 #include <stdint.h>
-#include <stdio.h> // BUFSIZ
 #include <string>
 #include <variant>
 #include <vector>
 
 #include "platform.hpp" // SSIZE_MAX
 
-#define LEXER_BUF_SIZE BUFSIZ
+// This value is a compromise between `LexerState` allocation performance when `mmap` works, and
+// buffering performance when it doesn't/can't (e.g. when piping a file into RGBASM).
+#define LEXER_BUF_SIZE 64
 // The buffer needs to be large enough for the maximum `lexerState->peek()` lookahead distance
 static_assert(LEXER_BUF_SIZE > 1, "Lexer buffer size is too small");
 // This caps the size of buffer reads, and according to POSIX, passing more than SSIZE_MAX is UB