v0.7.1c: switched to experimental R8 optimizer which replaces proguard.

A few things to note:
- R8 is currently experimental, but is becoming default in AS 3.4
- R8 results in a decent decrease in bytecode size
- R8 necessitates switching off proguard-optimize, as that config nukes
  the line number table even with -keepattributes LineNumberTable.
  Hopefully that will be fixed and optimize can be re-enabled for
  even more size savings.
- No instability has been observed, but if any crops up I will revert

core/build.gradle

@@ -18,7 +18,7 @@ android {
         release {
             shrinkResources true
             minifyEnabled true
-            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
+            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
         }
     }
 }