Add stdlib test importing every stdlib module

[jiribenes]

Resolves #614.

I'm pretty sure this will reveal at least two bugs. :)
EDIT: Yep, three different libraries didn't even compile. 😠
EDIT 2: Aaaand, there are two different miscomputations on LLVM! 😆 So 5 bugs altogether so far.

ACME in our context stands for "All Common Modules in Effekt" :)

[jiribenes]

Ideal follow-up: make CI so that on change in /libraries/common/, we check that the acme.effekt file contains every module in that folder, recursively (just generate it again and diff). If not, fail and spam the PR.
EDIT: Done. ACME CI now exists.

[jiribenes]

New LLVM bugs, cc @phischu:

$ EFFEKT_DEBUG=1 effekt --backend=llvm --ir-write-all --debug --optimize examples/stdlib/json.effekt
opt: ./out/json.ll:4001:9: error: multiple definition of local value named 'd_24_132_3_22162'
        %d_24_132_3_22162 = insertvalue %Neg %vtable_temporary_865, %Object %closure_861, 1
        ^

$ EFFEKT_DEBUG=1 effekt --backend=llvm --ir-write-all --debug --optimize examples/stdlib/buffer.effekt
Instruction does not dominate all uses!
  %buffer_24_4387 = insertvalue %Neg %vtable_temporary_1257, ptr %closure_1239, 1
  call void @shareNegative(%Neg %buffer_24_4387)
Instruction does not dominate all uses!
  %buffer_24_4387 = insertvalue %Neg %vtable_temporary_1257, ptr %closure_1239, 1
  store %Neg %buffer_24_4387, ptr %buffer_24_4387_pointer_1255, align 8, !noalias !0
Instruction does not dominate all uses!
  %buffer_24_4387 = insertvalue %Neg %vtable_temporary_1257, ptr %closure_1239, 1
  call void @eraseNegative(%Neg %buffer_24_4387)
opt: ./out/buffer.ll: error: input module is broken!

[jiribenes]

I think this is semantically correct, we use it as a circular buffer, so we don't really need to invalidate the previous value -- we'd have overwritten it.