More work for Rust binding

[chenyan-dfinity]

Part of #255

• Generated names conform to Rust convention: Pascal case for type names and enum tags; snake case for function names.
• Fix problems for agent code
• Fix a bug when deriving empty struct/tuple enum tag, e.g., #[derive(CandidType)] enum T { A{}, B() }.

[github-actions]

Benchmark for 695d419

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Test	Base	PR	%
Blob/&str	291.0±136.73µs	286.1±144.14µs	-1.68%
Blob/ByteBuf	212.9±54.76µs	212.3±58.18µs	-0.28%
Blob/Bytes	141.2±44.77µs	142.4±49.22µs	+0.85%
Blob/String	356.5±155.49µs	350.9±172.24µs	-1.57%
Collections/vec (text, nat)	81.4±3.74ms	81.3±3.88ms	-0.12%
Collections/vec int	35.7±1.47ms	37.2±2.36ms	+4.20%
Collections/vec int64	20.1±1.22ms	19.9±0.98ms	-1.00%
Collections/vec nat8	15.4±0.59ms	15.3±0.69ms	-0.65%
option list/1024	1626.7±77.58µs	1615.5±67.31µs	-0.69%
profiles/1024	3.1±0.12ms	3.0±0.11ms	-3.23%
variant list/1024	1277.9±58.06µs	1277.7±53.48µs	-0.02%

[github-actions]

Benchmark for e154897

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Test	Base	PR	%
Blob/&str	200.8±76.27µs	196.2±63.96µs	-2.29%
Blob/ByteBuf	142.5±48.31µs	140.4±50.04µs	-1.47%
Blob/Bytes	98.9±35.15µs	102.2±34.71µs	+3.34%
Blob/String	213.0±75.59µs	218.9±76.40µs	+2.77%
Collections/vec (text, nat)	62.2±0.83ms	62.9±0.83ms	+1.13%
Collections/vec int	30.0±0.09ms	30.2±0.08ms	+0.67%
Collections/vec int64	17.3±0.31ms	17.5±0.23ms	+1.16%
Collections/vec nat8	13.2±0.08ms	13.1±0.08ms	-0.76%
option list/1024	1332.4±2.17µs	1367.0±41.55µs	+2.60%
profiles/1024	2.6±0.00ms	2.6±0.00ms	0.00%
variant list/1024	1137.8±4.62µs	1135.7±4.10µs	-0.18%

[github-actions]

Benchmark for 8765f1f

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Test	Base	PR	%
Blob/&str	242.3±75.73µs	216.6±58.65µs	-10.61%
Blob/ByteBuf	143.9±48.34µs	144.9±47.58µs	+0.69%
Blob/Bytes	101.8±44.07µs	101.7±44.03µs	-0.10%
Blob/String	313.6±155.72µs	303.3±144.38µs	-3.28%
Collections/vec (text, nat)	65.0±0.87ms	65.0±1.10ms	0.00%
Collections/vec int	32.5±0.09ms	32.2±0.03ms	-0.92%
Collections/vec int64	16.7±0.08ms	17.4±0.13ms	+4.19%
Collections/vec nat8	14.3±0.01ms	14.2±0.01ms	-0.70%
option list/1024	1383.3±1.40µs	1390.6±2.30µs	+0.53%
profiles/1024	2.7±0.04ms	2.7±0.03ms	0.00%
variant list/1024	1133.1±1.57µs	1124.7±0.86µs	-0.74%

[github-actions]

Benchmark for c46c63a

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Test	Base	PR	%
Blob/&str	249.1±70.08µs	238.1±68.58µs	-4.42%
Blob/ByteBuf	158.4±43.70µs	160.3±42.75µs	+1.20%
Blob/Bytes	116.2±41.88µs	116.4±41.29µs	+0.17%
Blob/String	316.2±145.72µs	308.7±145.00µs	-2.37%
Collections/vec (text, nat)	69.3±0.75ms	69.3±1.32ms	0.00%
Collections/vec int	33.0±0.06ms	33.5±0.05ms	+1.52%
Collections/vec int64	18.4±0.14ms	19.8±0.15ms	+7.61%
Collections/vec nat8	14.4±0.02ms	14.3±0.03ms	-0.69%
option list/1024	1396.2±2.97µs	1395.0±3.33µs	-0.09%
profiles/1024	2.7±0.01ms	2.7±0.00ms	0.00%
variant list/1024	1137.0±1.59µs	1125.0±1.39µs	-1.06%