Microptimization in implicit search

Avoid creating a throwaway list of parameter types.
Merge pull request #3084 from retronym/topic/opt

Microoptimization in implicit search
deduplicate tuple tree creation code

Previously tuple -> tree representation has been implemented in
three place: tree builder, tree gen, build utils. Now it's just
defined once in tree gen.
wip

Conflicts:
	src/compiler/scala/tools/nsc/ast/parser/TreeBuilder.scala