lang: add set_inner function to Account<'a, T>

[paul-schaaf]

closes #1115

pub fn new_user(ctx: Context<CreateUser>, new_user:User) -> ProgramResult {
    ctx.accounts.user_to_create.set_inner(new_user);
}

instead of

pub fn new_user(ctx: Context<CreateUser>, new_user:User) -> ProgramResult {
    (*ctx.accounts.user_to_create).name = new_user.name;
    (*ctx.accounts.user_to_create).age = new_user.age;
    (*ctx.accounts.user_to_create).address = new_user.address;
}