Implement path stringify (#1387)

Ref https://github.com/svg/svgo/issues/32

Added `stringifyPathData` utility to produce small as possible path by
- matching leading moveto and line to commands (m -> l, M -> L)
- combining moveto and lineto commands
- avoiding space before decimal numbers if possible

lib/path.js

@@ -208,3 +208,81 @@ const parsePathData = (string) => {
   return pathData;
 };
 exports.parsePathData = parsePathData;
+
+const stringifyNumber = ({ number, precision }) => {
+  let result;
+  if (precision == null) {
+    result = number.toString();
+  } else {
+    result = number.toFixed(precision);
+    if (result.includes('.')) {
+      result = result.replace(/\.?0+$/, '')
+    }
+  }
+  // remove zero whole from decimal number
+  if (result !== '0') {
+    result = result.replace(/^0/, '').replace(/^-0/, '-');
+  }
+  return result;
+};
+
+// elliptical arc large-arc and sweep flags are rendered with spaces
+// because many non-browser environments are not able to parse such paths
+const stringifyArgs = ({ args, precision }) => {
+  let result = '';
+  let prev;
+  for (let i = 0; i < args.length; i += 1) {
+    const number = args[i];
+    const numberString = stringifyNumber({ number, precision });
+    // avoid space before first and negative numbers
+    if (i === 0 || numberString.startsWith('-')) {
+      result += numberString;
+    } else if (prev.includes('.') && numberString.startsWith('.')) {
+      result += numberString;
+    } else {
+      result += ` ${numberString}`;
+    }
+    prev = numberString;
+  }
+  return result;
+};
+
+const stringifyPathData = ({ pathData, precision }) => {
+  // combine sequence of the same commands
+  let combined = [];
+  for (let i = 0; i < pathData.length; i += 1) {
+    const { command, args } = pathData[i];
+    if (i === 0) {
+      combined.push({ command, args });
+    } else {
+      const last = combined[combined.length - 1];
+      // match leading moveto with following lineto
+      if (i === 1) {
+        if (command === 'L') {
+          last.command = 'M';
+        }
+        if (command === 'l') {
+          last.command = 'm';
+        }
+      }
+      if (
+        (last.command === command &&
+          last.command !== 'M' &&
+          last.command !== 'm') ||
+        // combine matching moveto and lineto sequences
+        (last.command === 'M' && command === 'L') ||
+        (last.command === 'm' && command === 'l')
+      ) {
+        last.args = [...last.args, ...args];
+      } else {
+        combined.push({ command, args });
+      }
+    }
+  }
+  let result = '';
+  for (const { command, args } of combined) {
+    result += command + stringifyArgs({ args, precision });
+  }
+  return result;
+};
+exports.stringifyPathData = stringifyPathData;