Bump forc to 0.6

[adlerjohn]

Probably requires #13 to be done simultaneously.

[mitchmindtree]

Also requires FuelLabs/sway#917 (along with a follow-up forc 0.6.1 release) for the tests to pass as witnessed by @vnepveu (see here).

[iqdecay]

After trying to update forc to 0.6, it seems some tests are broken for no reason:

---- abigen_different_structs_same_arg_name stdout ----
  Bytecode size is 156 bytes.
Contract deployed @ 9906afc4d317e5d16ff3694bd14c0f908f1a5c21a685f6e778e18b2b363d73da
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `43`', fuels-abigen-macro/tests/harness.rs:1337:5

If you look a the concerned contract, there is afaik no reason its behavior should be changed by the forc update.
There are several other cases of broken tests as Mitch mentioned in FuelLabs/sway#915

[digorithm]

@vnepveu https://github.com/FuelLabs/fuels-rs/blob/master/fuels-core/src/code_gen/functions_gen.rs#L117-L135

We're still checking for the 3 mandatory arguments (u64, u64, b256) and encoding it in the call. forc/sway 0.6.x removed it, so if you compile it with forc 0.6.x, it will be deployed with all methods not including these args in the function signature but the SDK will still be encoding it, leading to all sorts of errors.

I'm currently working on these changes to support forc 0.6 though.