Unrefactor peek() to use tail recursion instead of a loop

Profiling RGBDS on building pret/pokecrystal reveals commit
cfe1f60e47 as a clear outlier
among the past few hundred commits for reducing performance
(an 8% increase on my machine from ~5 to ~5.4 seconds).

src/asm/lexer.cpp

@@ -720,14 +720,17 @@ static std::shared_ptr<std::string> readInterpolation(size_t depth);
 static int peek() {
 	int c = lexerState->peekChar();
 
-	for (; lexerState->macroArgScanDistance == 0; c = lexerState->peekChar()) {
+	if (lexerState->macroArgScanDistance > 0) {
+		return c;
+	}
+
 	++lexerState->macroArgScanDistance; // Do not consider again
 
 	if (c == '\\' && !lexerState->disableMacroArgs) {
 		// If character is a backslash, check for a macro arg
 		++lexerState->macroArgScanDistance;
 		if (!isMacroChar(lexerState->peekCharAhead())) {
-				break;
+			return c;
 		}
 
 		// If character is a macro arg char, do macro arg expansion
@@ -740,16 +743,19 @@ static int peek() {
 			// https://en.wikipedia.org/wiki/Painted_blue
 			lexerState->macroArgScanDistance += str->length();
 		}
+
+		return peek(); // Tail recursion
 	} else if (c == '{' && !lexerState->disableInterpolation) {
 		// If character is an open brace, do symbol interpolation
 		shiftChar();
 		if (std::shared_ptr<std::string> str = readInterpolation(0); str) {
 			beginExpansion(str, *str);
 		}
-		}
-	}
 
+		return peek(); // Tail recursion
+	} else {
 		return c;
+	}
 }
 
 static void shiftChar() {