feat!: 💥 dimensionless quantities refactored

Dimensionless quantities are now represented by quantity types rather
than by plain representation types. Only dimensionless quantities with
`unitless` unit are implicitly convertible from representation types.

`units::exp()` now is a function doing std::exp() on a representation
type (previous `units::exp` class template was renamed to
`units::exponent`).

BREAKING_CHANGE: gcc-9.3 support removed
BREAKING_CHANGE: `exp` and `Exp` renamed to `exponent` and `Exponent`
Resolves #27
Resolves #42

src/include/units/exponent.h

@@ -0,0 +1,85 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <units/base_dimension.h>
+#include <units/ratio.h>
+
+namespace units {
+
+/**
+ * @brief A power of factor corresponding to the dimension of a quantity
+ *
+ * @tparam Dim component dimension of a derived quantity
+ * @tparam Num numinator of the factor
+ * @tparam Den denominator of the factor
+ */
+template<Dimension Dim, std::intmax_t Num, std::intmax_t Den = 1>
+struct exponent {
+  using dimension = Dim;
+  static constexpr int num = Num;
+  static constexpr int den = Den;
+};
+
+// is_exponent
+namespace detail {
+
+template<typename Dim, std::intmax_t Num, std::intmax_t Den>
+inline constexpr bool is_exponent<exponent<Dim, Num, Den>> = true;
+
+}  // namespace detail
+
+// exp_less
+template<Exponent E1, Exponent E2>
+  requires BaseDimension<typename E1::dimension> && BaseDimension<typename E2::dimension>
+struct exp_less : base_dimension_less<typename E1::dimension, typename E2::dimension> {};
+
+// exp_invert
+namespace detail {
+
+template<typename Dim, std::intmax_t Num, std::intmax_t Den>
+constexpr exponent<Dim, -Num, Den> exp_invert_impl(exponent<Dim, Num, Den>);
+
+}  // namespace detail
+
+template<Exponent E>
+using exp_invert = decltype(detail::exp_invert_impl(E()));
+
+// exp_multiply
+namespace detail {
+
+template<Exponent E, std::intmax_t Num, std::intmax_t Den>
+struct exp_multiply_impl {
+  static constexpr ratio r = ratio(E::num, E::den) * ratio(Num, Den);
+  using type = exponent<typename E::dimension, r.num, r.den>;
+};
+
+}  // namespace detail
+
+template<Exponent E, std::intmax_t Num, std::intmax_t Den>
+using exp_multiply = TYPENAME detail::exp_multiply_impl<E, Num, Den>::type;
+
+template<Exponent... Es>
+struct exp_list {};
+
+}  // namespace units