Make ast.Node return *nature.Nature

[diegommm]

Fix #839

We keep using nature.Nature instead of *nature.Nature inside of ast.base because this reduces the number of checks and makes the implementation safer. Also, we always assume that we will have a nature, and the zero value of a nature is also useful, so no need to ever return nil.

@antonmedv this actually resulted in a minor performance improvement in several places and it also makes code more readable as you suggested 😃

Checker benchmarks

goos: linux
goarch: amd64
pkg: github.com/expr-lang/expr/checker
cpu: 13th Gen Intel(R) Core(TM) i7-13700H   
                                            │ bench-results-old.txt │      bench-results-stable.txt      │
                                            │        sec/op         │   sec/op     vs base               │
Checker/name=function_calls-20                          4.658µ ± 3%   4.620µ ± 2%       ~ (p=0.218 n=10)
Checker/name=unary_and_binary_operations-20             4.512µ ± 2%   4.502µ ± 1%       ~ (p=0.697 n=10)
Checker/name=deep_struct_access-20                      12.31µ ± 1%   12.17µ ± 1%  -1.11% (p=0.002 n=10)
geomean                                                 6.371µ        6.326µ       -0.71%

                                            │ bench-results-old.txt │      bench-results-stable.txt       │
                                            │         B/op          │    B/op     vs base                 │
Checker/name=function_calls-20                         0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Checker/name=unary_and_binary_operations-20            0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Checker/name=deep_struct_access-20                     0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                                           ²               +0.00%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean   

                                            │ bench-results-old.txt │      bench-results-stable.txt       │
                                            │       allocs/op       │ allocs/op   vs base                 │
Checker/name=function_calls-20                         0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Checker/name=unary_and_binary_operations-20            0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Checker/name=deep_struct_access-20                     0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                                           ²               +0.00%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean