Refs #454 -- Added VariablePacking to general/build/smart-contracts/g…

…as-optimization (#487)

* update general/conflux-basics/glossary.md, regarding the issue #455

* Update docs/general/conflux-basics/glossary.md

Co-authored-by: darwintree <[email protected]>

* Update glossary.md

* add packing in gas-optimization

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Co-authored-by: darwintree <[email protected]>

docs/general/build/smart-contracts/gas-optimization/packing.md

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+---
+displayed_sidebar: generalSidebar
+---
+
+# Variable Packing
+
+The Ethereum Virtual Machine (EVM) stores variables in consecutive 32-bytes slots. When we place multiple variables within a single slot, this is referred to as variable packing.
+
+If the variables we try to pack exceed the 32-bytes limit of the current slot, they will be stored in a new slot. It's crucial to determine which variables are best grouped together to minimize wasted space. 
+
+Although Solidity automatically tries to pack smaller basic types into the same slot, poor struct member ordering can prevent the compiler from doing so.
+
+Learn More: [Layout of State Variables in Storage](https://docs.soliditylang.org/en/v0.8.25/internals/layout_in_storage.html)
+
+**DemoCode**
+
+Below, we demonstrate how to use packing in contracts to compare gas usage.
+
+```solidity
+
+// gas: 188616
+contract Standard {
+    uint64 a = 5;
+    uint256 b = 5;
+    uint64 c = 5;
+}
+
+// gas: 166178
+contract OptimizedPacking {
+    uint256 b = 5;
+    uint64 a = 5;
+    uint64 c = 5;
+}
+```
+
+Recommendations for gas optimization:
+
+🌟Pay attention to variable packing when choosing data types. If it's possible to pack a variable with others into a single storage slot, opting for a smaller data type can be beneficial.