Swift 5. Updated pods. Moved sign functions to Keychain

[ypopovych]

Updated PR with compatibility with Swift 5 only.

Also moved EthereumPrivateKey and EthereumPublicKey to own package. For someone who wants to use the library with own signature/key management parts. Enabled by default for CocoaPods.

[codecov[bot]]

Codecov Report

Merging #77 into master will increase coverage by 0.2%. The diff coverage is 68.55%.

@@            Coverage Diff            @@
##           master      #77     +/-   ##
=========================================
+ Coverage   75.14%   75.35%   +0.2%     
=========================================
  Files          64       64             
  Lines        4152     4187     +35     
=========================================
+ Hits         3120     3155     +35     
  Misses       1032     1032

Impacted Files	Coverage Δ
Web3/Classes/Keychain/Secp256k1+CTXCreator.swift	68.42% <ø> (ø)
Web3/Classes/PromiseKit/Promisable.swift	100% <ø> (ø)	:arrow_up:
...s/Core/Json/EthereumTransactionReceiptObject.swift	0% <0%> (ø)	:arrow_up:
...3/Classes/ContractABI/Contract/SolidityEvent.swift	64.44% <0%> (ø)	:arrow_up:
Web3/Classes/ContractABI/ABI/Eth+ABI.swift	0% <0%> (ø)	:arrow_up:
Web3/Classes/Core/Json/EthereumLogObject.swift	0% <0%> (ø)	:arrow_up:
Web3/Classes/Core/Toolbox/BytesConvertible.swift	0% <0%> (ø)	:arrow_up:
...Classes/Core/Toolbox/String+BytesConvertible.swift	78.57% <0%> (ø)	:arrow_up:
Web3/Classes/ContractABI/Contract/ERC165.swift	0% <0%> (ø)	:arrow_up:
.../Classes/Core/Json/EthereumTransactionObject.swift	50% <0%> (-22.23%)	:arrow_down:
... and 35 more

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[ypopovych]

all tests passed

[ypopovych]

@Ybrin @pixelmatrix guys, can we start the process of merging? I will need the updated library in my own libraries, and I don't want to fork it. It's better to update yours.

[pixelmatrix]

The Swift 5 stuff looks good to me. My only suggestion there is that Hashable is able to be computed for many custom structs now, so many of the custom hash(into:) methods may be unnecessary.

Personally I'd prefer this PR only introduces a single change (Swift 5 OR Keychain changes), but this is much cleaner than the previous one. I'm not sure how @Ybrin will feel about the Keychain stuff and splitting it into more submodules, but this seems fine to me.

[ypopovych]

About the logic behind Keychain extraction.

We have our own keychain implementation (with Rust based HD wallet). And in current realization, Web3 adds a dependency to secp256k1.swift, which we will never use. If we will split it, it can be used in both ways.

[ypopovych]

@Ybrin @pixelmatrix any news?

[koraykoska]

@ypopovych