ERC-1996 Holdable Token

StagnantERCcreated 2019-04-103 later proposals build on itrequires ERC-20

Authors: Julio Faura, Fernando Paris, Daniel Lehrner

ERC-165 interface ids

Computed here as the XOR of the selectors each interface defines.

IHoldable 0x54e79bec

`0x7649c6d1`	`authorizeHoldOperator(address)`
`0xc5a468c6`	`balanceOnHold(address)`	view
`0xda7d3d04`	`executeHold(string,uint256)`
`0x78404999`	`hold(string,address,address,uint256,uint256)`
`0xc5b670f0`	`holdFrom(string,address,address,address,uint256,uint256)`
`0xc2ac5b5c`	`isHoldOperatorFor(address,address)`	view
`0x0b986a50`	`netBalanceOf(address)`	view
`0x8e4a5a92`	`releaseHold(string)`
`0xec38b229`	`renewHold(string,uint256)`
`0x4cb0267f`	`retrieveHoldData(string)`	view
`0x36c8773f`	`revokeHoldOperator(address)`
`0x51bfb961`	`totalSupplyOnHold()`	view

Topic0 is the keccak256 hash of the signature, which is what a log filter matches on.

`0x8a1d1765ef42a1fb3d5b0e12ba4b1083e020407cb5ba8e07aaaacf610624aec9`	`HoldCreated(address,string,address,address,address,uint256,uint256)`
`0x5c325a8e2c888924f4a27940863613323b3a56cfbbbf8ee144e180ab228e104c`	`HoldExecuted(address,string,address,uint256,uint256)`
`0x4e48b247593fd0b3174316619f9274ea86db0ed03e691c2f8c4e0400e923375e`	`HoldReleased(address,string,uint8)`
`0xab224ddedc192922a9caa9c23294ca7c6c25bd489024c2d08e16b8aaffce2486`	`HoldRenewed(address,string,uint256,uint256)`
`0xbb785bb78fa937a58ce1ce2c3a4bfdaf457b51bdedc3944e94c26d6777c7214a`	`AuthorizedHoldOperator(address,address)`
`0x2e96d672cd86b559c08b9503df494f90c7766b7804ed5c650d0adde07cdf483f`	`RevokedHoldOperator(address,address)`

Proposals this one names in its requires field.

Later proposals that name this one in their requires field.