Switch to the mimalloc allocator

[ZeWaka]

Switches the langserver from the default system allocator to mimalloc.
Gives a great speedup in parse time and a decrease in memory usage.

Mimalloc is more designed for the tiny short-lived allocations we're doing with the parser.
I chose v2 because v3 actually uses more memory. I looked into some other allocators but they didn't give much gains.

Mimalloc does hold onto freed pages until another alloc or asked, so we force a collect at the end of heavy operations where there wouldn't be another alloc (server goes idle).

Median of 3 runs each on my local Windows machine:

Codebase	Build	Parse	Load time (parse + refs + dreamchecker)	Private memory	Working set	Peak working set
goonstation	before	3.01 s	3.79 s	503 MB	482 MB	522 MB
goonstation	after	1.81 s (−40%)	2.35 s (−38%)	434 MB (−14%)	425 MB (−12%)	483 MB (−7%)
tgstation	before	4.07 s	5.23 s	659 MB	632 MB	691 MB
tgstation	after	2.38 s (−42%)	3.10 s (−41%)	566 MB (−14%)	553 MB (−13%)	633 MB (−8%)

Speedup is less on Linux (Well, WSL) because their allocator is less shite. More like 20% loading and memory reduction.

Sidenote:

I think even more gains could be had by splitting proc body parsing (about 50% of the used memory) out since the only things that use them are dreamchecker and Find References. Dropping it after dreamchecker and lazily reparsing on Find-Reference could be worth it. Yep, and didn't need to do reparsing lazily: #486

[ZeWaka]

Your choice if you want to mark #113 fixed by this (or a potential future proc body parsing split PR).

[SpaceManiac]

Benched on Linux, also including dreamchecker CLI. Improvements are big on musl, smaller but still meaningful on glibc.

codebase	libc	alloc	dc time (s)	ls parse (s)	ls total (s)	ls mem (K)
tgstation	musl	default	4.63	3.551	4.71	652060
tgstation	musl	mi	2.90 (-37%)	2.372 (-33%)	2.959 (-37%)	568260 (-13%)
tgstation	gnu	default	2.86	2.092	2.694	695404
tgstation	gnu	mi	2.36 (-17%)	1.795 (-14%)	2.275 (-16%)	568296 (-18%)
goonstation	musl	default	3.57	2.692	3.606	475632
goonstation	musl	mi	2.27 (-36%)	1.841 (-32%)	2.319 (-36%)	420664 (-12%)
goonstation	gnu	default	2.15	1.563	2.026	527452
goonstation	gnu	mi	1.83 (-15%)	1.366 (-13%)	1.740 (-14%)	420656 (-20%)

I might want to switch release builds off musl if I can! Here's the musl->glibc comparison:

codebase	alloc	dc time	ls parse	ls total	ls mem
tgstation	default	38%	41%	43%	-7%
tgstation	mi	19%	24%	23%	0%
goonstation	default	40%	42%	44%	-11%
goonstation	mi	19%	26%	25%	0%