[eval] Rework capture variable handling

[Simn]

I know I know, another typical Haxe-rc change. But I have to do it.

Here's the good news from the benchmarks:

what	before	after	change
static closure	3,937,595	4,128,581	+5%
field closure	3,352,771	3,492,588	+4%
local closure	2,595,895	3,172,965	+22%
local function	2,496,921	4,080,627	+63%

In particular, local functions that don't capture anything are now treated just like normal static functions, giving them a huge speedup.

closes #7983