Change count() return type to UInt64 #292

Job	Run time
build with wasm-pack	2m 40s
cargo test (macos)	5m 34s
Check cargo fmt	39s
cargo check	11m 5s
cargo test (macos-aarch64)	8m 54s
Verify MSRV (Min Supported Rust Version)	8m 4s
Verify Vendored Code	53s
cargo test (win64)	11m 21s
cargo test (amd64)	7m 22s
cargo test datafusion-cli (amd64)	4m 3s
check Cargo.toml formatting	2m 54s
clippy	5m 35s
cargo test hash collisions (amd64)	7m 8s
cargo doc	4m 8s
cargo examples (amd64)	14m 2s
cargo test doc (amd64)	15m 53s
Run sqllogictest with Postgres runner	3m 40s
cargo test pyarrow (amd64)	2m 47s
verify benchmark results (amd64)	12m 19s
check configs.md and ***_functions.md is up-to-date	3m 13s
	2h 12m 14s

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