TODO: Remove default template parameter values

Platforms: all

Coding time: S
Experimentation time: S
Skill required: S

We should generally not have default values for template parameters: this makes
code harder to read, and is discouraged by the Google C++ style guide for good
reason.

Specifically, I'm concerned about CellFormat having the CellOrder parameter
defaulting to WidthMajor. This specific case has been causing confusion.

There might be other instances to fix. Part of this TODO item is to audit that.

One exception in which default template parameters are OK, is for locally
contained metaprogramming helpers, that are a local implementation detail.
However, I don't know if we have any such case in gemmlowp. We're generally
conservative with template metaprogramming around here.
