Update features & some fixes

[Dzejkop]

Update features to be compatible with nightly rust.

Also fixes compilation errors that occurred if asm was enabled.

[shamatar]

What are those features exactly about? I can in principle try to work around them somehow with extra effort in procmacro, and then it will work in stable

[Dzejkop]

What are those features exactly about? I can in principle try to work around them somehow with extra effort in procmacro, and then it will work in stable

At some point the asm feature got split into these 2 the asm_sym and asm_const and they refer to operands sym <path> and const <expr> respectively.

The details are beyond me, but without this change I was unable to use this crate on the current version of Rust.

[citizen-stig]

@shamatar Hello!
Do you need any more details from @Dzejkop ?

Our project got around 30% performance improvement with this feature enabled, so this can be a big deal.