Add default values to compiler arguments

[elle-j]

Description

Adds default values to the compiler arguments below when parsed by clap. This does not change previous user-facing behavior as these defaults were implemented using different logic, but improves the UX and the visibility of the exact defaults used.

Example output from resolc --help:
(Only affected arguments are shown here, and some of their descriptions have been omitted here.)

Options:

  -O, --optimization <OPTIMIZATION>
          Set the optimization parameter -O[0 | 1 | 2 | 3 | s | z]. Use `3` for best performance and `z` for minimal size
          
          [default: z]

      --metadata-hash <METADATA_HASH>
          Set the metadata hash type. Available types: `none`, `ipfs`, `keccak256`
          
          [default: keccak256]

      --heap-size <HEAP_SIZE>
          The emulated EVM linear heap memory static buffer size in bytes.
          ...
          
          [default: 131072]

      --stack-size <STACK_SIZE>
          The contracts total stack size in bytes.
          ...
          
          [default: 131072]

Resolved Issues

Closes #446

[elle-j]

For reference and examples, see value_source and default_value.

[xermicus]

LGTM modulo some nits

[xermicus]

Closes #446