Add concat() builtin

[antonmedv]

No description provided.

[antonmedv]

goos: darwin
goarch: arm64
pkg: github.com/expr-lang/expr
                           │   old.txt   │              new.txt               │
                           │   sec/op    │   sec/op     vs base               │
_expr-8                      64.12n ± 0%   64.09n ± 0%       ~ (p=0.590 n=10)
_expr_reuseVm-8              44.22n ± 0%   43.93n ± 0%  -0.66% (p=0.001 n=10)
_len-8                       35.25n ± 0%   35.31n ± 0%  +0.17% (p=0.013 n=10)
_filter-8                    43.43µ ± 0%   43.41µ ± 0%       ~ (p=0.684 n=10)
_filterLen-8                 40.09µ ± 0%   40.10µ ± 0%       ~ (p=0.517 n=10)
_filterFirst-8               401.8n ± 2%   401.5n ± 0%       ~ (p=0.617 n=10)
_filterLast-8                695.0n ± 1%   689.8n ± 0%  -0.76% (p=0.000 n=10)
_filterMap-8                 4.790µ ± 0%   4.795µ ± 1%       ~ (p=0.078 n=10)
_arrayIndex-8                60.17n ± 0%   60.84n ± 0%  +1.11% (p=0.000 n=10)
_envStruct-8                 44.12n ± 0%   43.75n ± 0%  -0.83% (p=0.000 n=10)
_envMap-8                    47.48n ± 0%   47.59n ± 0%  +0.23% (p=0.002 n=10)
_callFunc-8                  351.7n ± 0%   352.9n ± 0%  +0.36% (p=0.000 n=10)
_callMethod-8                363.9n ± 0%   365.1n ± 0%  +0.33% (p=0.000 n=10)
_callField-8                 58.49n ± 0%   58.52n ± 0%       ~ (p=0.105 n=10)
_callFast-8                  60.43n ± 0%   60.47n ± 0%       ~ (p=0.117 n=10)
_callConstExpr-8             47.60n ± 0%   47.80n ± 0%  +0.42% (p=0.000 n=10)
_largeStructAccess-8         122.6n ± 0%   123.3n ± 0%  +0.57% (p=0.003 n=10)
_largeNestedStructAccess-8   132.5n ± 0%   132.9n ± 0%  +0.30% (p=0.001 n=10)
_largeNestedArrayAccess-8    390.9µ ± 1%   393.3µ ± 1%       ~ (p=0.165 n=10)
_sort-8                      4.120µ ± 0%   4.252µ ± 0%  +3.22% (p=0.000 n=10)
_sortBy-8                    9.331µ ± 0%   9.478µ ± 0%  +1.58% (p=0.000 n=10)
_groupBy-8                   9.924µ ± 0%   9.869µ ± 0%  -0.55% (p=0.000 n=10)
_reduce-8                    4.596µ ± 0%   4.614µ ± 0%  +0.39% (p=0.000 n=10)
geomean                      597.0n        598.7n       +0.28%