better handling for add then removed or changed symbols

[mgerdts]

There are times when symbols change over time. For instance:

        /*
         * V8_SCOPEINFO_IDX_FIRST_VARS' value was 4 in V8 3.7 and up,
         * then 5 when StrongModeFreeVariableCount was added with
         * https://codereview.chromium.org/1005063002, and 6 when
         * ContextGlobalCount was added with
         * https://codereview.chromium.org/1218783005.
         * Since the current V8_CONSTANT_FALLBACK macro doesn't allow
         * us to specify different values for different V8 versions,
         * these are hardcoded below.
         */

Rather than having one case defined in v8_constants or v8_objects and others defined in a series of if statements, it should be possible to mark each v8_constant_t or v8_offset_t with an "add" version and a "remove" version (rather than just one of them). With this, it would be possible to cleanly handle changes using multiple v8_constant_t or v8_object_t elements for the same symbol and non-overlapping versions.