Fixed semiauto derivation of Default instances

[joroKr21]

Achieved via refactoring of DefaultMacros.materialize.

Fixes #747

[joroKr21]

Some missing functionality in Scala 2.10 makes things more verbose. Depending on how #655 turns out, DefaultMacros can be simplified at a later point (probably true for other macros as well).

[codecov-io]

Codecov Report

Merging #764 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #764   +/-   ##
=======================================
  Coverage   86.96%   86.96%           
=======================================
  Files          90       90           
  Lines        2517     2517           
  Branches       53       53           
=======================================
  Hits         2189     2189           
  Misses        328      328

Impacted Files	Coverage Δ
core/src/main/scala/shapeless/default.scala	0% <ø> (ø)	⬆️
core/src/test/scala/shapeless/default.scala	100% <100%> (ø)	⬆️

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[milessabin]

LGTM! Many thanks ... merging :-)

[milessabin]

Although Travis showed this as green previously, rerunning the build after clearing the build caches results in a build failure on 2.10.x: https://travis-ci.org/milessabin/shapeless/builds/274500921.

Would you mind taking a look?

[joroKr21]

This looks like a spurious error. I saw it in #765 but I couldn't reproduce it locally. The failing example has nothing to do with Default. This commit fixes the build: a531c48

P.s. May or may not be related to #776

[milessabin]

Could you pull that commit out as a separate PR?