Refactor promotion.hpp (no policy promotion changes)

[ryanelandt]

Edit: #1021 was closed because it introduced misguided policy promotion changes. This PR contains the ideas from #1021 that are not problematic.

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This PR includes the following features of #1021

• Use of variadic templates instead of manual emulation with optional float arguments
• Use of std::common_type for float_type promotion simplification

This PR does not including the following features of #1021

• Support for 16 bit floating types

The file promotion.hpp in this PR is identical to that in #1021 except for line 91. In #1021 this reads:

using type = std::common_type_t<T1, T2>;

In this PR it reads:

using type = std::common_type_t<T1, T2, float>;

This preserves the current floating point promotion behavior.

[mborland]

Merged in with 1179