scalar is no longer needed, now that
interfaces always hold pointers.

Comparing best of 5 with TurboBoost turned off,
on a 2012 Retina MacBook Pro Core i5.
Still not completely confident in these numbers,
but the gob and template improvements seem real.

benchmark                       old ns/op   new ns/op   delta
BenchmarkBinaryTree17           3819892491  3803008185  -0.44%
BenchmarkFannkuch11             3623876405  3611776426  -0.33%
BenchmarkFmtFprintfEmpty        119         118         -0.84%
BenchmarkFmtFprintfString       294         292         -0.68%
BenchmarkFmtFprintfInt          310         304         -1.94%
BenchmarkFmtFprintfIntInt       513         507         -1.17%
BenchmarkFmtFprintfPrefixedInt  427         426         -0.23%
BenchmarkFmtFprintfFloat        562         554         -1.42%
BenchmarkFmtManyArgs            1873        1832        -2.19%
BenchmarkGobDecode              15824504    14746565    -6.81%
BenchmarkGobEncode              14347378    14208743    -0.97%
BenchmarkGzip                   537229271   537973492   +0.14%
BenchmarkGunzip                 134996775   135406149   +0.30%
BenchmarkHTTPClientServer       119065      116937      -1.79%
BenchmarkJSONEncode             29134359    28928099    -0.71%
BenchmarkJSONDecode             106867289   105770161   -1.03%
BenchmarkMandelbrot200          5798475     5791433     -0.12%
BenchmarkGoParse                5299169     5379201     +1.51%
BenchmarkRegexpMatchEasy0_32    195         195         +0.00%
BenchmarkRegexpMatchEasy0_1K    477         477         +0.00%
BenchmarkRegexpMatchEasy1_32    170         170         +0.00%
BenchmarkRegexpMatchEasy1_1K    1412        1397        -1.06%
BenchmarkRegexpMatchMedium_32   336         337         +0.30%
BenchmarkRegexpMatchMedium_1K   109025      108977      -0.04%
BenchmarkRegexpMatchHard_32     5854        5856        +0.03%
BenchmarkRegexpMatchHard_1K     184914      184748      -0.09%
BenchmarkRevcomp                829233526   836598734   +0.89%
BenchmarkTemplate               142055312   137016166   -3.55%
BenchmarkTimeParse              598         597         -0.17%
BenchmarkTimeFormat             564         568         +0.71%

Fixes #7425.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant, khr
https://golang.org/cl/158890043