Add trait "iscomplete" to Alphabet

[jakobnissen]

Add trait "iscomplete" to every alphabet. It should default to Val{false}(). It should return Val{true} if length(symbols(A)) == 1 << bits_per_symbol(A), i.e. if all bitpatterns are valid. In that case, the decoder can skip validation. There are also other efficiencies to be had, such as better random sequence generation.

[TransGirlCodes]

Oooh I like that idea!

[jakobnissen]

Okay, this is now added in #161 . Turns out it was not that important, actually. With a little tweaking of the index functions to remove all unnecessary branches and signed/unsigned conversions, here is what f(x, i) = @inbounds x[i] compiles to:

julia> @code_native debuginfo=:none f(a, 2)
	.section	__TEXT,__text,regular,pure_instructions
	leaq	-4(,%rsi,4), %rax
	movq	(%rdi), %rcx
	movq	(%rcx), %rcx
	movq	%rax, %rdx
	shrq	$3, %rdx
	andq	$-8, %rdx
	shrxq	%rax, (%rcx,%rdx), %rax
	andb	$15, %al
	retq
	nopw	%cs:(%rax,%rax)

🎉