feat: add typed memory accessors to Wasmtime::Memory

[takahashim]

Motivation

Currently, to retrieve an integer value from linear memory, we need to write something like memory.read(out_p, 4).unpack1("l<").
This approach is verbose, difficult to understand, and prone to errors.
Therefore, we want to enable writing memory.read_i32(out_p) instead.

Changes

Introduce paired read/write methods for the WebAssembly numeric types (little-endian):

• #read_i32 / #write_i32
• #read_u32 / #write_u32
• #read_i64 / #write_i64
• #read_u64 / #write_u64
• #read_f32 / #write_f32
• #read_f64 / #write_f64

Out-of-bounds access raises Wasmtime::Error, consistent with #read / #write.

[saulecabrera]

For future PRs involving changes to the public API, please open an issue first for discussion. wasmtime-rb's minimal API surface is intentional, we try to keep it as close as upstream as possible. In this case, the change is small enough that I think it is reasonable to land.