std.objectRemoveKey panics when given extended objects

[ushitora-anqou]

The following code panics

std.objectRemoveKey({}+{}, "")

with the following error

❯ jsonnet --version        
Jsonnet commandline interpreter (Go implementation) v0.21.0

❯ cat misc/bug1.jsonnet 
std.objectRemoveKey({}+{}, "")

❯ jsonnet misc/bug1.jsonnet
INTERNAL ERROR: (CRASH) interface conversion: jsonnet.uncachedObject is *jsonnet.extendedObject, not *jsonnet.simpleObject
goroutine 1 [running]:
runtime/debug.Stack()
	runtime/debug/stack.go:26 +0x5e
github.com/google/go-jsonnet.(*VM).Evaluate.func1()
	github.com/google/go-jsonnet/vm.go:187 +0x3d
panic({0x5e95a0?, 0xc0001b8900?})
	runtime/panic.go:792 +0x132
github.com/google/go-jsonnet.builtInObjectRemoveKey(0xc000190500, {0x683f60, 0xc0001b8870}, {0x683ee8, 0xc00000ef30})
	github.com/google/go-jsonnet/builtins.go:2512 +0x3e6
github.com/google/go-jsonnet.(*binaryBuiltin).evalCall(0xc0000e50e0, {{0xc000049a40, 0x2, 0x2}, {0x9ea4e0, 0x0, 0x0}, 0x0}, 0xc000190500)
	github.com/google/go-jsonnet/builtins.go:2625 +0x20e
github.com/google/go-jsonnet.(*valueFunction).call(0xc000049590, 0xc000190500, {{0xc000049a40, 0x2, 0x2}, {0x9ea4e0, 0x0, 0x0}, 0x0})
	github.com/google/go-jsonnet/value.go:349 +0x122
github.com/google/go-jsonnet.(*interpreter).evaluateTailCall(0x685628?, 0xc000054100?, {{0xc000049a40, 0x2, 0x2}, {0x9ea4e0, 0x0, 0x0}, 0x0}, 0x1)
	github.com/google/go-jsonnet/interpreter.go:1037 +0x9d
github.com/google/go-jsonnet.(*interpreter).rawevaluate(0xc000190500, {0x685628, 0xc000054100}, 0x1)
	github.com/google/go-jsonnet/interpreter.go:621 +0x3be5
github.com/google/go-jsonnet.(*interpreter).evaluate(0xc000190500, {0x685628, 0xc000054100}, 0x1)
	github.com/google/go-jsonnet/interpreter.go:312 +0x4e
github.com/google/go-jsonnet.(*interpreter).EvalInCleanEnv(0xc000190500, 0x11?, {0x685628, 0xc000054100}, 0x10?)
	github.com/google/go-jsonnet/interpreter.go:1019 +0x5f
github.com/google/go-jsonnet.evaluateAux(0xc000190500, {0x685628, 0xc000054100}, 0xc0000e5dd0)
	github.com/google/go-jsonnet/interpreter.go:1327 +0x147
github.com/google/go-jsonnet.evaluate({0x685628, 0xc000054100}, 0xc0001ddd68?, 0xc0000e5dd0, 0x5ea920?, 0xc0000e5f20?, 0x7ffe65d92fdf?, {0x6838c0?, 0xc0000b2040?}, 0x0, ...)
	github.com/google/go-jsonnet/interpreter.go:1362 +0x91
github.com/google/go-jsonnet.(*VM).Evaluate(0xc0001b8000?, {0x685628?, 0xc000054100?})
	github.com/google/go-jsonnet/vm.go:190 +0xba
github.com/google/go-jsonnet.(*VM).EvaluateFile(0xc0001b0230, {0x7ffe65d92fdf?, 0xc0000e5cb0?})
	github.com/google/go-jsonnet/vm.go:419 +0xb4
main.main()
	github.com/google/go-jsonnet/cmd/jsonnet/cmd.go:457 +0x6c5

Please report a bug here: https://github.com/google/go-jsonnet/issues

Seemingly this line crashed:
https://github.com/google/go-jsonnet/blob/v0.21.0/builtins.go#L2512

We need to take care of the situation where obj.uncached isn't a simpleObject.