Standard Library reorganization

[nfurfaro]

As the stdlib grows, patterns of organization and usage are beginning to emerge.
Currently the library is primarily structured around what things are rather than how they are used.
I propose we switch to a more developer-focused API, where we group things together that are typically used in the same context.
Below is an example of how we could potentially add contracts.sw& calls.sw modules following this new organizing principle:

//!Functionality that is either logically related to or only usable in the context of contracts
library contracts;


// currently in context.sw
pub fn balance_of(...) -> u64;
pub fn this_balance() -> u64

// currently in call_frames.sw
pub fn contract_id() -> ContractId

// currently in token.sw
pub fn mint_to(...)
pub fn mint_to_contract(...)
pub fn mint_to_address(...)
pub fn mint(...)
pub fn burn(...)
pub fn transfer(...)
pub fn force_transfer_to_contract(...)
pub fn transfer_to_address(...)

//!Functionality that is either logically related to or usable only in the context of contract calls
library calls;


/** 
NOTE: Solidity exposes these as things like `msg.sender`, `msg.amount`, `msg.data`. 
There may be some confusion with our `message` types (`Input::Message` & `Output::Message`) if we 
continue to follow this naming precedent. Similar to the proposed `Contract` type with methods on it, 
we could introduce a `Call` type with methods, i.e:
let amount = call.amount();
let sender = call.sender();
etc...
*/


// currently in call_frames.sw
pub fn msg_asset_id() -> ContractId

// currently in context.sw
pub fn msg_amount() -> u64;
pub fn gas() -> u64

// currently in auth.sw
pub fn msg_sender() -> Result<Identity, AuthError>