Fix compatibility with basicnode.Prototype.Any.NewBuilder()

[i-norden]

Trying to use the basicnode.Prototype.Any.NewBuilder() instead of a specifically typed builder from our typeSlab (e.g. dageth.Type.TrieNode.NewBuilder()) causes a panic to be thrown, e.g. while working on #47

--- FAIL: TestEthCodecs (0.00s)
    --- FAIL: TestEthCodecs/eth-log-trie-branch-node-with-embedded-log-leaf-and-value (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x1012521f0]

goroutine 174 [running]:
testing.tRunner.func1.2({0x1013ebb80, 0x10173c680})
	/usr/local/go/src/testing/testing.go:1396 +0x1c8
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1399 +0x378
panic({0x1013ebb80, 0x10173c680})
	/usr/local/go/src/runtime/panic.go:884 +0x204
github.com/vulcanize/go-codec-dageth.(*_Value__KeyAssembler).AssignString(0x140004997f8?, {0x10130bb57, 0x3})
	/Users/iannorden/go/src/github.com/vulcanize/go-codec-dageth/ipldsch_satisfaction.go:27306 +0x70
github.com/vulcanize/go-codec-dageth.(*_Value__KeyAssembler).AssignNode(0x14000245160?, {0x101475dd8?, 0x14000245180?})
	/Users/iannorden/go/src/github.com/vulcanize/go-codec-dageth/ipldsch_satisfaction.go:27322 +0x58
github.com/vulcanize/go-codec-dageth.(*_Value__Assembler).AssignNode(0x140002e7bc8, {0x101475d38, 0x14000245160})
	/Users/iannorden/go/src/github.com/vulcanize/go-codec-dageth/ipldsch_satisfaction.go:27079 +0x37c
github.com/vulcanize/go-codec-dageth.(*_TrieBranchNode__Assembler).AssignNode(0x140002e7818, {0x101475d38, 0x14000245120})
	/Users/iannorden/go/src/github.com/vulcanize/go-codec-dageth/ipldsch_satisfaction.go:19108 +0x36c
github.com/vulcanize/go-codec-dageth.(*_TrieNode__Assembler).AssignNode(0x140002e7800, {0x101475d38, 0x140002450e0})
	/Users/iannorden/go/src/github.com/vulcanize/go-codec-dageth/ipldsch_satisfaction.go:23216 +0x3a0
github.com/vulcanize/go-codec-dageth/trie.AppendEncode({0x14000284000, 0x0, 0x400}, {0x101475d38, 0x140002450e0})
	/Users/iannorden/go/src/github.com/vulcanize/go-codec-dageth/trie/marshal.go:70 +0x134
github.com/vulcanize/go-codec-dageth/trie.Encode({0x101475d38, 0x140002450e0}, {0x128f87a60, 0x14000272e00})
	/Users/iannorden/go/src/github.com/vulcanize/go-codec-dageth/trie/marshal.go:56 +0x50
github.com/vulcanize/go-codec-dageth/log_trie.Encode({0x101475d38?, 0x140002450e0?}, {0x128f87a60?, 0x14000272e00?})
	/Users/iannorden/go/src/github.com/vulcanize/go-codec-dageth/log_trie/marshal.go:16 +0x2c
github.com/ipld/codec-fixtures/go.nodeToCid({0x14000258de0, 0x14000258df0, 0x1014681e8, 0x0, 0x0, 0x0, 0x0, 0x0}, {0x10146b008, 0x140000244c0}, ...)
	/Users/iannorden/go/src/github.com/ipld/codec-fixtures/go/fixtures.go:242 +0x150
github.com/ipld/codec-fixtures/go.verifyCid(0x14000327040, {0x1400022b300, 0x3a}, {0x101475d38?, 0x140002450e0?}, {0x10146b008?, 0x140000244c0?}, {{0x14000289e00?, 0x5000?}}, {0x14000258de0, ...})
	/Users/iannorden/go/src/github.com/ipld/codec-fixtures/go/codecs_test.go:76 +0x9c
github.com/ipld/codec-fixtures/go.TestEthCodecs.func1(0x14000327040)
	/Users/iannorden/go/src/github.com/ipld/codec-fixtures/go/codecs_test.go:68 +0x2b8
testing.tRunner(0x14000327040, 0x14000296e40)
	/usr/local/go/src/testing/testing.go:1446 +0x10c
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1493 +0x300
exit status 2
FAIL	github.com/ipld/codec-fixtures/go	0.179s

This appears to only occur for Union types. I believe this related to #19 and #20, but need to investigate further (and close out those issues as well, in any case).