Improve accessibility

[mingxwa]

Changes

• Revised the semantics of accessor. Let accessors defined in a dispatch be aware of the overload types to generate better APIs. This also improves diagnostics and code completion from a modern IDE. Invoking a proxy will no longer cause confusion in IntelliSense (tested with the latest version of Visual Studio and VS Code). Resolves Unexpected intellisense error message in VS2022 #103
• Revised the design of operator dispatches. Previously, operator dispatches are defined as "prefix" and "postfix". After this change, this concept no longer applies because some operators (specifically, +, -, ++, --, * and &) requires the operator be defined in a same type to avoid ambiguity. Therefore, we have decided to divide the definition of operator dispatches in a new way: whether it applies to left-hand-side operand (LHS) or right-hand-side operand (RHS). As a result, for most cases (except for the 6 operators mentioned above), PRO_DEF_POSTFIX_OPERATOR_DISPATCH is replaced by PRO_DEF_LHS_OPERATOR_DISPATCH; PRO_DEF_PREFIX_OPEATOR_DISPATCH is replaced by PRO_DEF_RHS_OPERATOR_DISPATCH.
• Added __declspec(empty_bases) for MSVC to improve code generation for accessors. Covered by unit tests. Resolves Having two or more indirect conventions will make proxy 8 bytes larger than expected in MSVC #123
• Implemented RHS version of 9 assignment operators (+=, -=, *=, /=, &=, |=, ^=, <<=, >>=), which was missing before this change. Covered by unit tests.