JMESPath slices and pre-computed runtime expressions (#1510)

* More JMESPath unit tests

* move detail::from_chars to parse.hpp

* Supporting slices

* slicing working

* run-time slice working

* multi-bracket access

* jmespath_expression

* Update JMESPath.md

Author: stephenberry

docs/JMESPath.md

@@ -72,3 +72,77 @@ suite jmespath_read_at_tests = [] {
 };
 ```
 
+## Run-time Expressions
+
+It can be expensive to tokenize at runtime. The runtime version of `glz::read_jmespath` takes in a `const glz::jmespath_expression&`. This allows expressions to be pre-computed, reused, and cached for better runtime performance.
+
+```c++
+Person child{};
+// A runtime expression can be pre-computed and saved for more efficient lookups
+glz::jmespath_expression expression{"family.children[0]"};
+expect(not glz::read_jmespath(expression, child, buffer));
+expect(child.first_name == "Lilly");
+```
+
+Note that this still works:
+
+```c++
+expect(not glz::read_jmespath("family.children[0]", child, buffer));
+```
+
+## Slices
+
+```c++
+suite jmespath_slice_tests = [] {
+   "slice compile-time"_test = [] {
+      std::vector<int> data{0,1,2,3,4,5,6,7,8,9};
+      std::string buffer{};
+      expect(not glz::write_json(data, buffer));
+      
+      std::vector<int> slice{};
+      expect(not glz::read_jmespath<"[0:5]">(slice, buffer));
+      expect(slice.size() == 5);
+      expect(slice[0] == 0);
+      expect(slice[1] == 1);
+      expect(slice[2] == 2);
+      expect(slice[3] == 3);
+      expect(slice[4] == 4);
+   };
+   
+   "slice run-time"_test = [] {
+      std::vector<int> data{0,1,2,3,4,5,6,7,8,9};
+      std::string buffer{};
+      expect(not glz::write_json(data, buffer));
+      
+      std::vector<int> slice{};
+      expect(not glz::read_jmespath("[0:5]", slice, buffer));
+      expect(slice.size() == 5);
+      expect(slice[0] == 0);
+      expect(slice[1] == 1);
+      expect(slice[2] == 2);
+      expect(slice[3] == 3);
+      expect(slice[4] == 4);
+   };
+   
+   "slice compile-time multi-bracket"_test = [] {
+      std::vector<std::vector<int>> data{{1,2},{3,4,5},{6,7}};
+      std::string buffer{};
+      expect(not glz::write_json(data, buffer));
+      
+      int v{};
+      expect(not glz::read_jmespath<"[1][2]">(v, buffer));
+      expect(v == 5);
+   };
+   
+   "slice run-time multi-bracket"_test = [] {
+      std::vector<std::vector<int>> data{{1,2},{3,4,5},{6,7}};
+      std::string buffer{};
+      expect(not glz::write_json(data, buffer));
+      
+      int v{};
+      expect(not glz::read_jmespath("[1][2]", v, buffer));
+      expect(v == 5);
+   };
+};
+```
+