Implementation of toEthAddress function

[vaniiiii]

Description

This pull request adds the required functionality related to issue #4

Developers can utilize this function to convert a f4 FileAddress type into an EthAddress.

Approach and implementation

The starting point was PR #16 which already included the implementation of this function. However, unit tests for various types of addresses were missing. The missing unit tests are added while retaining the existing fuzz tests with minor changes.

Validate function changes

The validate function checks the FilAddress in bytes format, which includes a protocol byte and payload.

The previous implementation was failing for valid input because it was comparing the bytes length with 10:

if (addr.data[0] == 0x00) {
    return addr.data.length <= 10; // should be <= 11 
}

This approach is incorrect, as the f0 format contains a protocol byte and payload, which can be a maximum of 10 bytes long. An example is provided in the Filecoin spec document, in the last example in section 6.1.3.1.

|----------|---------------|
| protocol |    payload    |
|----------|---------------|
|    0     | leb128-varint |

Discussion

• Should the validate function also include checks for minimum length for f0 and f4 addresses? For example, should 0x00 be considered a valid f0 address?
• Should f4 addresses be checked against a length of 65 if the payload is hardcoded to 54, as specified in FIP. If the payload is not 64 - prefix.len(), the total length, according to the format, should be 65 bytes. More details can be found in this discussion

|------------|----------------|---------------------|
|  protocol  |      actor     |      payload        | 
|------------|----------------|---------------------|
|     04     |  leb128-variant|      54 bytes       |