Gradle API Scanner plugin (#1669)

* Skeleton plugin.
* Implement Gradle api-scanner plugin, and apply it.
* Generate API documentation for any jar without a classifier.
* Fix usage of smokeTests classifier.
* Tweak Gradle API usage.
* Upgrade to fast-classpath-scanner 2.7.0
* Include interfaces and more modifiers in the class description.
* Allow system classes to be supertypes and implemented interfaces.
* Make API Scanner plugin configuration tweakable via build.gradle.
* Add a miserable amount of unit testing.
* Sort methods and fields using their natural comparators. Way easier!
* Add README for api-scanner plugin.
* Add @OutputFiles to ScanApiTask.
* Rename ScanApiTask to ScanApi.
* Allow the ScanApi task to be disabled.
* WIP: Create a top-level GenerateApi task to collate the ScanApi output.
* Exclude package-private classes, as well as bridge/synthetic methods.
* Replace "End of Class" delimiter with '##'.
* Don't scan modules whose API is still "in flux".
* Include constructors in the API definitions.
* Finish implementation of GenerateApi task.
* Update README to include GenerateApi task.
* Filter out Kotlin's "internal" methods.
* Assign "fatjar" classifier to the fat jar artifact.
* Enhance README for GenerateApi.
* Explain effect of api-scanner plugin, and link to Corda's API strategy.
* Tweak README
* Exclude synthetic Kotlin classes by analysing @Metadata.
* Allow us to exclude some classes explicitly from the API.

core/build.gradle

@@ -2,6 +2,7 @@ apply plugin: 'kotlin'
 apply plugin: 'kotlin-jpa'
 apply plugin: 'net.corda.plugins.quasar-utils'
 apply plugin: 'net.corda.plugins.publish-utils'
+apply plugin: 'net.corda.plugins.api-scanner'
 apply plugin: 'com.jfrog.artifactory'
 
 description 'Corda core'
@@ -94,6 +95,13 @@ jar {
     baseName 'corda-core'
 }
 
+scanApi {
+    excludeClasses = [
+        // Kotlin should probably have declared this class as "synthetic".
+        "net.corda.core.Utils\$toFuture\$1\$subscription\$1"
+    ]
+}
+
 publish {
     name jar.baseName
 }