Improve the performance of runtime.Fetch for structures

[montag451]

The improvement comes from the use of a cache to speed up the retrieval of struct fields. This commit also adds a new benchmark to measure the performance gain.

goos: linux
goarch: amd64
pkg: github.com/expr-lang/expr
cpu: Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
                   │ /tmp/old.txt │            /tmp/new.txt             │
                   │    sec/op    │   sec/op     vs base                │
_envStruct_noEnv-8    503.3n ± 4%   228.8n ± 3%  -54.53% (p=0.000 n=10)

                   │ /tmp/old.txt │            /tmp/new.txt            │
                   │     B/op     │    B/op     vs base                │
_envStruct_noEnv-8     48.00 ± 0%   32.00 ± 0%  -33.33% (p=0.000 n=10)

                   │ /tmp/old.txt │            /tmp/new.txt            │
                   │  allocs/op   │ allocs/op   vs base                │
_envStruct_noEnv-8     3.000 ± 0%   1.000 ± 0%  -66.67% (p=0.000 n=10)