Only convert struct names to camel case for Call variant structs

[ascjones]

Fixes #410.

Unfortunately there are no tests (yet) for the Call struct codegen from variants so that aspect is not tested here.

[fewensa]

I have a question for there. Why convert struct names to snake case for Call variant structs? This is not rust code style

[ascjones]

Call variants are for a pallet are in snake case, so e.g. Balances::transfer_keep_alive, and from that we create a standalone struct TransferKeepAlive. Technically I think it is called PascalCase with the first letter capitalised but it seems to work that way in heck. Now that I mention it, I think we need to update heck because it is now called to_upper_camel_case: https://docs.rs/heck/latest/heck/trait.ToUpperCamelCase.html

[fewensa]

Currently, All Call variant structs generated use snake case. it's ugly.

            #[derive(:: subxt :: codec :: Encode, :: subxt :: codec :: Decode, Debug, Clone)]
            pub struct as_multi {
                pub threshold: ::core::primitive::u16,
                pub other_signatories: ::std::vec::Vec<::subxt::sp_core::crypto::AccountId32>,
                pub maybe_timepoint: ::core::option::Option<
                    runtime_types::pallet_multisig::Timepoint<::core::primitive::u32>,
                >,
                pub call: ::std::vec::Vec<::core::primitive::u8>,
                pub store_call: ::core::primitive::bool,
                pub max_weight: ::core::primitive::u64,
            }
            impl ::subxt::Call for as_multi {
                const PALLET: &'static str = "Multisig";
                const FUNCTION: &'static str = "as_multi";
            }
            #[derive(:: subxt :: codec :: Encode, :: subxt :: codec :: Decode, Debug, Clone)]
            pub struct approve_as_multi {
                pub threshold: ::core::primitive::u16,
                pub other_signatories: ::std::vec::Vec<::subxt::sp_core::crypto::AccountId32>,
                pub maybe_timepoint: ::core::option::Option<
                    runtime_types::pallet_multisig::Timepoint<::core::primitive::u32>,
                >,
                pub call_hash: [::core::primitive::u8; 32usize],
                pub max_weight: ::core::primitive::u64,
            }
            impl ::subxt::Call for approve_as_multi {
                const PALLET: &'static str = "Multisig";
                const FUNCTION: &'static str = "approve_as_multi";
            }

[ascjones]

This is a bug, they should be UpperCamelCase

[ascjones]

#441