Improve documentation and remove unnecessary generics in the userspace

[StackOverflowExcept1on]

Problem to Solve

• some functions such as gstd::exec::system_reserve_gas are re-exported from gcore and when user goes to the documentation, all code examples shows usage of gcore but not gstd
• replace <E: Encode> with just impl Encode in gstd

Possible Solution

• https://docs.rs/about/metadata, #[cfg(feature = ...)] #[doc = "..."]
• not that D: Decode is still useful for msg::send_for_reply_as

Notes

No response