ng/parser: support type-elided slice constructions

[sbinet]

the following fails in ng:

ng> type T struct { Name string }
ng> ts := []T{{Name:"name"}}
neugram: parser: panic: TODO parseStmt } (off 54)
goroutine 20 [running]:
runtime/debug.Stack(0x32, 0x0, 0x0)
	/home/binet/sdk/go/src/runtime/debug/stack.go:24 +0xa7
runtime/debug.PrintStack()
	/home/binet/sdk/go/src/runtime/debug/stack.go:16 +0x22
neugram.io/ng/parser.(*Parser).work.func1(0xc42013a070)
	/home/binet/work/gonum/src/neugram.io/ng/parser/parser.go:145 +0x116
panic(0xaf4b00, 0xc42001e720)
	/home/binet/sdk/go/src/runtime/panic.go:502 +0x229
neugram.io/ng/parser.(*Parser).parseStmt(0xc42013a070, 0xc42001e6e0, 0x1)
	/home/binet/work/gonum/src/neugram.io/ng/parser/parser.go:1233 +0x1836
neugram.io/ng/parser.(*Parser).work(0xc42013a070)
	/home/binet/work/gonum/src/neugram.io/ng/parser/parser.go:188 +0x1f4
created by neugram.io/ng/parser.New
	/home/binet/work/gonum/src/neugram.io/ng/parser/parser.go:30 +0x114
ng eval panic: expr TODO: &expr.Bad{
	Position: src.Pos{
		Filename: "/home/binet/work/gonum/src/neugram.io/ng/ng-interactive",
		Line: int32(2),
		Column: int16(11),
	},
	Error: parser.Error{
		Pos: src.Pos{
			Filename: "/home/binet/work/gonum/src/neugram.io/ng/ng-interactive",
			Line: int32(2),
			Column: int16(12),
		},
		Offset: int(45),
		Msg: "expected operand, got ident",
	},
}
goroutine 1 [running]:
runtime/debug.Stack(0x186, 0x0, 0x0)
	/home/binet/sdk/go/src/runtime/debug/stack.go:24 +0xa7
runtime/debug.PrintStack()
	/home/binet/sdk/go/src/runtime/debug/stack.go:16 +0x22
neugram.io/ng/eval.(*Program).Eval.func1(0xc42012c180, 0xc420137630, 0xc420137618)
	/home/binet/work/gonum/src/neugram.io/ng/eval/eval.go:476 +0x1ea
panic(0xaf4b00, 0xc42001e890)
	/home/binet/sdk/go/src/runtime/panic.go:502 +0x229
neugram.io/ng/typecheck.(*Checker).exprPartial(0xc42014e1b0, 0xc442c0, 0xc420099080, 0x1, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc442c0, ...)
	/home/binet/work/gonum/src/neugram.io/ng/typecheck/typecheck.go:2679 +0x59aa
neugram.io/ng/typecheck.(*Checker).expr(0xc42014e1b0, 0xc442c0, 0xc420099080, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/home/binet/work/gonum/src/neugram.io/ng/typecheck/typecheck.go:1350 +0x79
neugram.io/ng/typecheck.(*Checker).checkSliceLiteral(0xc42014e1b0, 0xc44780, 0xc42009f860, 0x0, 0x0, 0x0, 0xc42001e6c0, 0x1, 0x1, 0xc42001e6a0, ...)
	/home/binet/work/gonum/src/neugram.io/ng/typecheck/typecheck.go:2811 +0x24a
neugram.io/ng/typecheck.(*Checker).exprPartial(0xc42014e1b0, 0xc44780, 0xc42009f860, 0x0, 0x3, 0xc40a00, 0xc42001e6a0, 0x0, 0x0, 0xc44780, ...)
	/home/binet/work/gonum/src/neugram.io/ng/typecheck/typecheck.go:2218 +0x4fa0
neugram.io/ng/typecheck.(*Checker).exprNoElide(0xc42014e1b0, 0xc44780, 0xc42009f860, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/home/binet/work/gonum/src/neugram.io/ng/typecheck/typecheck.go:1359 +0x79
neugram.io/ng/typecheck.(*Checker).stmt(0xc42014e1b0, 0xc448c0, 0xc42009f8b0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/home/binet/work/gonum/src/neugram.io/ng/typecheck/typecheck.go:166 +0x378
neugram.io/ng/typecheck.(*Checker).Add(0xc42014e1b0, 0xc448c0, 0xc42009f8b0, 0x0, 0x0)
	/home/binet/work/gonum/src/neugram.io/ng/typecheck/typecheck.go:3490 +0x94
neugram.io/ng/eval.(*Program).Eval(0xc42012c180, 0xc448c0, 0xc42009f8b0, 0xc4201680c0, 0x0, 0x0, 0x0, 0xc3fc00, 0xc42001e8a0)
	/home/binet/work/gonum/src/neugram.io/ng/eval/eval.go:481 +0xe2
main.handleResult(0x2, 0xc42000c4e0, 0x2, 0x2, 0x0, 0x0, 0x0, 0xc4200c4180, 0x4, 0x4)
	/home/binet/work/gonum/src/neugram.io/ng/ng.go:393 +0x72b
main.loop(0xc400000000)
	/home/binet/work/gonum/src/neugram.io/ng/ng.go:385 +0x5d2
main.main()
	/home/binet/work/gonum/src/neugram.io/ng/ng.go:147 +0x5be
ng: ng eval panic: expr TODO: &expr.Bad{
	Position: src.Pos{
		Filename: "/home/binet/work/gonum/src/neugram.io/ng/ng-interactive",
		Line: int32(2),
		Column: int16(11),
	},
	Error: parser.Error{
		Pos: src.Pos{
			Filename: "/home/binet/work/gonum/src/neugram.io/ng/ng-interactive",
			Line: int32(2),
			Column: int16(12),
		},
		Offset: int(45),
		Msg: "expected operand, got ident",
	},
}
name
neugram: parser: expected operand, got ident (off 45)
neugram: parser: expected "}", found "ident" (off 45)
neugram: parser: expected ";", found ":" (off 46)
neugram: parser: panic: TODO parseStmt } (off 54)

of course, this works, though:

ng> type T struct { Name string }
ng> ts := []T{T{Name:"name"}}
ng> ts
[]struct { Name string }{
	struct { Name string }{
		Name: "name",
	},
}