Change count() return type to UInt64 #288

Job	Run time
cargo check	11m 24s
cargo test (macos)	5m 16s
build with wasm-pack	2m 38s
Check cargo fmt	41s
cargo test (macos-aarch64)	7m 57s
Verify MSRV (Min Supported Rust Version)	8m 35s
Verify Vendored Code	54s
cargo test (win64)	11m 18s
cargo test (amd64)	6m 59s
cargo doc	4m 1s
cargo test datafusion-cli (amd64)	3m 49s
cargo test doc (amd64)	15m 53s
cargo examples (amd64)	13m 57s
Run sqllogictest with Postgres runner	3m 39s
cargo test pyarrow (amd64)	2m 48s
verify benchmark results (amd64)	12m 39s
check Cargo.toml formatting	3m 0s
check configs.md and ***_functions.md is up-to-date	2m 58s
clippy	5m 54s
cargo test hash collisions (amd64)	7m 4s
	2h 11m 24s

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