Compiler: Add "Top 10 slowest modules" to --stats output

[z64]

May be useful for spotting modules in large apps that may benefit from being broken up in order to improve build times, etc.

Example output:

Codegen (bc+obj):
 - 1757/1771 .o files were reused

These modules were not reused:
 - _main (_main.bc)
 - Process (P-rocess.bc)
 - /home/lune/git/crystal/src/compiler/crystal/macros/methods.cr (47home47lune47git-047426b817c2f879b1b5b500a3d97bbe.bc)
 - Crystal::Signal (C-rystal5858S-ignal.bc)
 - Crystal::Compiler (C-rystal5858C-ompiler.bc)
 - Hash(String, Int32) (H-ash40S-tring4432I-nt3241.bc)
 - Log::AsyncDispatcher (L-og5858A-syncD-ispatcher.bc)
 - Crystal::Command (C-rystal5858C-ommand.bc)
 - Crystal::Init (C-rystal5858I-nit.bc)
 - Crystal::Playground::Server (C-rystal5858P-layground5858S-erver.bc)
 - Crystal::Playground::Session (C-rystal5858P-layground5858S-ession.bc)
 - Crystal::Playground::PathWebSocketHandler (C-rystal5858P-lay-38c77a69c1a2ffd144caae7845092122.bc)
 - HTTP::Server (H-T-T-P-5858S-erver.bc)
 - HTTP::WebSocketHandler+ (H-T-T-P-5858W-ebS-ocketH-andler43.bc)

Top 10 slowest modules:
 - 00:00:00.794907504 _main (_main.bc)
 - 00:00:00.201288353 Crystal::Command (C-rystal5858C-ommand.bc)
 - 00:00:00.135537059 /home/lune/git/crystal/src/compiler/crystal/macros/methods.cr (47home47lune47git-047426b817c2f879b1b5b500a3d97bbe.bc)
 - 00:00:00.135338039 Crystal::Generic (C-rystal5858G-eneric.bc)
 - 00:00:00.111936502 Crystal::Compiler (C-rystal5858C-ompiler.bc)
 - 00:00:00.104652722 Crystal::ASTNode+ (C-rystal5858A-S-T-N-ode43.bc)
 - 00:00:00.091841367 Crystal::CodeGenVisitor (C-rystal5858C-odeG-enV-isitor.bc)
 - 00:00:00.083189850 Crystal::Call (C-rystal5858C-all.bc)
 - 00:00:00.075213115 Crystal::Type+ (C-rystal5858T-ype43.bc)
 - 00:00:00.066328602 Crystal::NumberLiteral (C-rystal5858N-umberL-iteral.bc)

[HertzDevil]

unit is a reference, so I believe direct mutation works and there is no need to do it like a value (unless there is some complication with forking)

[z64]

Pages in forked processes are copy-on-write by default. And the pages allocated by bdwgc are not marked as MEM_SHARED (probably for good reason, I'd wager). That is why this pipe mechanism is used, as a safe vehicle to share memory.

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Edit: Sorry, mistook the question; Yes - in this context we are in the parent thread, so mutating should be fine here.

[HertzDevil]

Are you still going to address this?

[z64]

@HertzDevil No, make any changes you see fit.

[HertzDevil]

This should probably be a property! instead