OpenSystemsDevelopment / qpdf

… checking, and align with `Array` API conventions.

…prove bounds checking

…y, modify bounds checks, and adjust affected tests.

… to use subscript operators, and simplify object access logic.

…hecks with `Array` subscript operator, simplify logic, and improve null handling.

…rray`, use subscript operators, simplify logic, and improve readability.

…s`, remove redundant trace, and improve null handling.

…move redundant trace, and improve readability.

…reamline object assignment, and introduce constructors for `QPDFObjectHandle`.

Implement subscript operators (`operator[]`) to provide indexed access to `QPDFObjectHandle` for both `size_t` and `int` types.

Refactor array handling in `QPDFObjectHandle`: replace `int` with `size_t` for size and index operations, introduce utility functions for type conversions, and simplify sparse array logic.

Move all array-specific methods to new class qpdf::Array.

Change the return type to a std::pair<bool, QPDFObjectHandle> in order to
allow a default constructed object handle (which is currently returned to
indicate failure) to become a valid object.

Create an alternative to getJSON to allow an object handle to be written as JSON without the overhead of creating a JSON object.

Sparse arrays are rare. Dynamically create the variables needed to manage
them only when needed.