Add pool_size method to MemoryPool (#218) (#230) #1426

Job	Run time
build with wasm-pack	1m 49s
cargo test (win64)	5m 42s
cargo check	1m 11s
cargo test (macos)	2m 24s
cargo test (macos-aarch64)	2m 30s
Check cargo fmt	32s
Verify MSRV (Min Supported Rust Version)	5m 24s
Verify Vendored Code	50s
cargo test (amd64)	0s
cargo test datafusion-cli (amd64)	0s
cargo doc	0s
cargo test doc (amd64)	0s
cargo examples (amd64)	0s
verify benchmark results (amd64)	0s
check Cargo.toml formatting	0s
clippy	0s
cargo test hash collisions (amd64)	0s
Run sqllogictest with Postgres runner	0s
cargo test pyarrow (amd64)	0s
check configs.md and ***_functions.md is up-to-date	0s
	20m 22s

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