pkg/list: add Haskell-style Take and Drop

In a post-slice world, these functions are a bit
more palatable than list.Slice if one of the
indices is open ended.

Adjusted slice signature and errors to be more
in line. Note that having less variation in error
messages reduces the burdon for applications that
would like to translate errors.

Change-Id: Id630d69aed9bfa88e71793a8068edb02987d6860
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3220
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/go.sum b/go.sum
index 6c216a5..db97c52 100644
--- a/go.sum
+++ b/go.sum
@@ -2,6 +2,7 @@
 github.com/cockroachdb/apd v1.1.0/go.mod h1:8Sl8LxpKi29FqWXR16WEFZRNSz3SoPzUzeMeY4+DwBQ=
 github.com/cockroachdb/apd/v2 v2.0.1 h1:y1Rh3tEU89D+7Tgbw+lp52T6p/GJLpDmNvr10UWqLTE=
 github.com/cockroachdb/apd/v2 v2.0.1/go.mod h1:DDxRlzC2lo3/vSlmSoS7JkqbbrARPuFOGr0B9pvN3Gw=
+github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/emicklei/proto v1.6.15 h1:XbpwxmuOPrdES97FrSfpyy67SSCV/wBIKXqgJzh6hNw=
 github.com/emicklei/proto v1.6.15/go.mod h1:rn1FgRS/FANiZdD2djyH7TMA9jdRDcYQ9IEN9yvjX0A=
@@ -38,6 +39,7 @@
 github.com/spf13/pflag v1.0.0/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
 github.com/spf13/pflag v1.0.3 h1:zPAT6CGy6wXeQ7NtTnaTerfKOsV6V6F8agHXFiazDkg=
 github.com/spf13/pflag v1.0.3/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
+github.com/stretchr/testify v1.2.0 h1:LThGCOvhuJic9Gyd1VBCkhyUXmO8vKaBFvBsJ2k03rg=
 github.com/stretchr/testify v1.2.0/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/xeipuuv/gojsonpointer v0.0.0-20170225233418-6fe8760cad35/go.mod h1:N2zxlSyiKSe5eX1tZViRH5QA0qijqEDrYZiPEAiq3wU=
 github.com/xeipuuv/gojsonreference v0.0.0-20150808065054-e02fc20de94c/go.mod h1:GwrjFmJcFw6At/Gs6z4yjiIwzuJ1/+UwLxMQDVQXShQ=