refactor(semver): compute formatted value lazily

This improves bench-parse results by up to 40%.

The external interface of the Range class is kept as-is except that the .version property is not writable anymore.

classes/semver.js

@@ -73,14 +73,20 @@ class SemVer {
     }
 
     this.build = m[5] ? m[5].split('.') : []
-    this.format()
+    this.formatted = undefined
   }
 
-  format () {
-    this.version = `${this.major}.${this.minor}.${this.patch}`
-    if (this.prerelease.length) {
-      this.version += `-${this.prerelease.join('.')}`
+  get version () {
+    if (this.formatted === undefined) {
+      this.formatted = `${this.major}.${this.minor}.${this.patch}`
+      if (this.prerelease.length) {
+        this.formatted += `-${this.prerelease.join('.')}`
+      }
     }
+    return this.formatted
+  }
+
+  format () {
     return this.version
   }
 
@@ -291,6 +297,7 @@ class SemVer {
       default:
         throw new Error(`invalid increment argument: ${release}`)
     }
+    this.formatted = undefined
     this.raw = this.format()
     if (this.build.length) {
       this.raw += `+${this.build.join('.')}`

functions/inc.js

@@ -9,7 +9,7 @@ const inc = (version, release, options, identifier, identifierBase) => {
 
   try {
     return new SemVer(
-      version instanceof SemVer ? version.version : version,
+      version instanceof SemVer ? version.raw : version,
       options
     ).inc(release, identifier, identifierBase).version
   } catch (er) {