Further reorganize typeclass implicits for faster implicit search

  - specific implicit for `Seq[T]`
  - make the implictits in the generated code by in a subclass
    of the low priority, expensive implicits.

After this change, we get to 187 discarded "expensive" implicits

    % scalac-hash v2.10.3 -Xlog-implicits  @args2.txt -Xprint:typer LME.scala 2>&1 | grep "not a valid" | wc -l

Down from 2200!

The benchmark compiles in 740ms, a 3.5x speedup from the original
2600ms.

project/GenWireFormat.scala

@@ -29,12 +29,12 @@ object GenWireFormat {
      val place = dir / "scoobi" / "codegen" / "GeneratedWireFormats.scala"
      IO.write(place,
 """package com.nicta.scoobi.codegen
-import com.nicta.scoobi.core.WireFormat
+import com.nicta.scoobi.core.{WireFormat, LowPriorityWireFormatImplicits}
 import java.io._
 
 object GeneratedWireFormats extends GeneratedWireFormats
 
-trait GeneratedWireFormats {
+trait GeneratedWireFormats extends LowPriorityWireFormatImplicits {
 """ + gen_section_1(22) + "\n\n" + gen_section_2(22) + "\n}")
 
      Seq(place)

src/main/scala/com/nicta/scoobi/core/WireFormat.scala

@@ -110,8 +110,10 @@ object WireFormat extends WireFormatImplicits {
 
 // Deprioritize expensive implicits to improve compile times.
 // See comments in `LowPriorityAvroSchemaImplicits`.
+//
+// Extended by `GeneratedWireFormats`
 trait LowPriorityWireFormatImplicits {
-  self: WireFormatImplicits =>
+  import WireFormat._
 
   /**
    * Traversable structures
@@ -260,7 +262,7 @@ trait LowPriorityWireFormatImplicits {
 }
 
 /** Implicit definitions of WireFormat instances for common types. */
-trait WireFormatImplicits extends LowPriorityWireFormatImplicits with codegen.GeneratedWireFormats {
+trait WireFormatImplicits extends codegen.GeneratedWireFormats {
 
   class ObjectWireFormat[T : Manifest](val x: T) extends WireFormat[T] {
     override def toWire(obj: T, out: DataOutput) {}

src/main/scala/com/nicta/scoobi/io/avro/AvroSchema.scala

@@ -67,8 +67,8 @@ trait AvroFixed[T] {
 // even if one of these was found to be eligible, it would be deemed lower priority
 // by the subclass rule of static overload resolution.
 //
-// These changes, together with corresponding changes in `WireFormat`, leads to a 2x
-// speedup (2600ms ~> 1300ms) in compiling a benchmark: 
+// These changes, together with corresponding changes in `WireFormat`, leads to a 3.5x
+// speedup (2600ms ~> 740ms) in compiling a benchmark: 
 // https://gist.github.com/retronym/fdef5a41c8e1e31124a4
 //
 // The goal is the reduce the number of lines like the following under `-Xlog-implicits`
@@ -193,6 +193,10 @@ object AvroSchema extends LowPriorityAvroSchemaImplicits {
     def toAvro(x: String): String = x
   }
 
+  // redundant, but better for performance to have this here rather than forcing
+  // this common implicit serach to go through TraversableSchema.
+  implicit def SeqSchema[T](implicit sch: AvroSchema[T]): AvroSchema[Seq[T]] = TraversableSchema[Seq, T]
+
   /* AvroSchema type class instance for Arrays. */
   implicit def ArraySchema[T](implicit mf: Manifest[T], sch: AvroSchema[T]) = new AvroSchema[Array[T]] {
     type AvroType = GenericData.Array[sch.AvroType]