ABI does not contain sufficient information to compute the correct function selector

## Description

The ABI does not contain sufficient information to compute the correct function selector for accessing the contract. More specifically, a `library` and a `contract` with the same entry points may have different 4 byte selectors, but nothing in the ABI allows to decide whether we are dealing with a contract or a libary.

A similar issue is raised in #7102, but there the conclusion is to put the information into the metadata, which does not the issue here. Possible solutions:

- Put a flag like `contractKind: contract` or `contractKind: library` into the ABI.
- For each function (and event?), add a field with the string that has to be hashed for the selector.
- Less preferably: for each function (and event?), add a field with the 4/32-byte selector.

## Environment

- Compiler version: 0.5.12
- Target EVM version (as per compiler settings): default
- Framework/IDE (e.g. Truffle or Remix): command line
- EVM execution environment / backend / blockchain client:
- Operating system: Linux

## Steps to Reproduce

The ABIs of the following two contracts are identical, but the selectors are different.
```
$ cat l.sol
pragma solidity ^0.5.0;
pragma experimental ABIEncoderV2;
library C {
struct S {address a;}
function F(S calldata) external pure returns (uint256) {return 0;}
}

$ solc --hashes --abi l.sol
[...]
Function signatures:
9b21e02d: F(C.S)
Contract JSON ABI
[{"constant":true,"inputs":[{"components":[{"internalType":"address","name":"a","type":"address"}],"internalType":"struct C.S","name":"","type":"tuple"}],"name":"F","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"payable":false,"stateMutability":"pure","type":"function"}]
```
```
$ cat c.sol
pragma solidity ^0.5.0;
pragma experimental ABIEncoderV2;
contract C {
struct S {address a;}
function F(S calldata) external pure returns (uint256) {return 0;}
}

$ solc --hashes --abi c.sol
[...]
Function signatures:
a0b61132: F((address))
Contract JSON ABI
[{"constant":true,"inputs":[{"components":[{"internalType":"address","name":"a","type":"address"}],"internalType":"struct C.S","name":"","type":"tuple"}],"name":"F","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"payable":false,"stateMutability":"pure","type":"function"}]
```

Start by reproducing the selector difference with l.sol and c.sol using solc --hashes --abi. Review the ABI design options in this issue and the related metadata discussion in #7102 before choosing a representation. Done means the emitted ABI lets consumers distinguish library and contract selectors, with coverage for both examples.

Written by the indexing model from the issue text.