Enhancements to ABI for Struct Generation

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This PR introduces several enhancements to the ABI related to the generation of structs. The main changes include:

• Struct Definition Entry and Exit: Two new functions, EnterStructDefinition and ExitStructDefinition, have been added. These functions are triggered when the parser enters and exits a struct definition, respectively. EnterStructDefinition appends a new struct to the ABI, while ExitStructDefinition prepares the struct for component resolution.
• Struct Appending: The AppendStruct function has been introduced. This function is called within EnterStructDefinition and is responsible for appending a new struct to the ABI.
• Struct Resolution: A new function, ResolveStructComponents, has been added. This function is called within ExitStructDefinition and is responsible for resolving the components of a struct, including handling nested structs and forward declarations.
• Struct Existence Check: A new function, isStructType, has been added. This function checks if a given type name corresponds to a defined struct. This helps in ensuring that only valid structs are processed further.

These changes aim to improve the robustness and clarity of the ABI generation process, particularly in relation to struct handling. Feedback and suggestions for further improvements are welcome.

[coveralls]

Pull Request Test Coverage Report for Build 5430741003

• 73 of 131 (55.73%) changed or added relevant lines in 3 files are covered.
• 42 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-10.7%) to 63.276%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
abi_listener.go	11	21	52.38%
abi_parser.go	58	106	54.72%

Files with Coverage Reduction	New Missed Lines	%
abi_listener.go	6	32.0%
abi_parser.go	36	62.22%

Totals
Change from base Build 5427608906:	-10.7%
Covered Lines:	367
Relevant Lines:	580

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💛 - Coveralls