[SM60 2/2] Update to Spidermonkey 60 APIs

Two noteworthy changes:

• Proxies are update to the SM60 API, having an explicit reserved slot and a private slot, in which the 'proxy data' and the C++ object are stored. This fixes a debug assertion failure of SM52 (See bugs 1237504 and 1339411)
• The GC callback behaviour has changed slightly, and we should now only look for GC_SLICE_BEGIN and GC_SLICE_END calls (Bug 1364547)

Other updates are minor:

• Bug 1339036: JSTYPE_VOID beomes JSTYPE_UNDEFINED
• Bug 1308236 - avoid ambiguous comparison by changing NULL to nullptr
• Bug 1421358, GC::reason::REFRESH_FRAME was removed. API is indicated in jsapi.h so use that.
• Compartment behaviours update
• ClassOps changes (Bug 1389510 removed the getter/setter - rP24229 - and bug 1370608 added one more before that so net minus one)
• Minor tests touchups again.

Tested by: SubitaNeo, Stan

Thanks to bellaz89 for the Shared Array fix

Closes #5859

Differential Revision: https://code.wildfiregames.com/D3116