Add system contract to convert tinybars to/from tinycents at active exchange rate

[tinker-michaelj]

Background
Suppose a contract aspires to be "self-funding"---that is, it expects users to send enough msg.value with function calls to cover the contract's Hedera fees.

The challenge is that although Hedera fees are fixed and predictable, they are denominated in tinycents (USD); but msg.value is denominated in tinybars. So the contract needs to convert from tinycents to tinybars, at the same exchange rate the network will use when charging fees.

Proposed solution
Add an EVM system contract at address 0x168 that implements the following interface,

interface IExchangeRate {
    // Given a value in tinycents (1e-8 US cents or 1e-10 USD), returns the 
    // equivalent value in tinybars (1e-8 HBAR) at the current exchange rate 
    // stored in system file 0.0.112. 
    // 
    // This rate is a weighted median of the the recent" HBAR-USD exchange 
    // rate on major exchanges, but should _not_ be treated as a live price 
    // oracle! It is important primarily because the network will use it to 
    // compute the tinybar fees for the active transaction. 
    // 
    // So a "self-funding" contract can use this rate to compute how much 
    // tinybar its users must send to cover the Hedera fees for the transaction.
    function tinycentsToTinybars(uint256 tinycents) external returns (uint256);

    // Given a value in tinybars (1e-8 HBAR), returns the equivalent value in 
    // tinycents (1e-8 US cents or 1e-10 USD) at the current exchange rate 
    // stored in system file 0.0.112. 
    // 
    // This rate tracks the the HBAR-USD rate on public exchanges, but 
    // should _not_ be treated as a live price oracle! This conversion is
    // less likely to be needed than the above conversion from tinycent to
    // tinybars, but we include it for completeness.
    function tinybarsToTinycents(uint256 tinybars) external returns (uint256);
}