Add named value accessors to constant_ and variable_

[ipapadop]

Motivation

All deferred objects currently use function-call syntax for evaluation:

auto value = object();

That is natural for composed deferred expressions, but less conventional for value wrappers such as constant_ and variable_. Standard C++ wrapper types usually expose named accessors such as value() or get().

Proposed API

Add named accessors while retaining operator() for uniform deferred evaluation:

constant.get();       // const T&
std::move(constant).get(); // T

variable.get();       // T&
std::as_const(variable).get(); // const T&
std::move(variable).get(); // T

value() is another possible name. get() aligns with std::reference_wrapper; value() aligns with std::optional and communicates value access more explicitly.

Design considerations

• Mirror the existing cv/ref-qualified return types exactly.
• Preserve noexcept and constexpr.
• Avoid duplicating implementation: either the named accessor or operator() should delegate to the other.
• Retain operator() so generic deferred evaluation and existing code remain unchanged.
• Decide whether mutating access through non-const variable_::get() is intentional and document it.
• Consider whether named access belongs only on constant_/variable_ or on every deferred expression. Restricting it to wrappers keeps the semantic distinction clear.

Compatibility

This can be a purely additive API change.

Acceptance criteria

• constant_ exposes a named accessor for const lvalues and rvalues.
• variable_ exposes named accessors for mutable lvalues, const lvalues, and rvalues.
• Return types match the corresponding existing operator() overloads.
• Accessors are constexpr and noexcept where the current operators are.
• Existing operator() behavior remains supported.
• Unit tests verify return types, mutation through variable_, and move access.
• Documentation explains named access versus deferred evaluation.