Notes: arenas, backend, linear types, interface in .so/.dll, stdlib implHEADmain

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+* safe&portable IR vs low-level IR
+    - lots of common stuff across ISAs.
+    - calling conventions (e.g. struct passing) can be complex and
+      it would be good to be able to re-use things that are common.
+    - some optimizations might be easier to do on the low-level IR
+      (and vice versa).
+* register alloc can be tricky:
+    - division on x86 is AFAIK limited to specific regs (eax,edx?)
+    - loop instructions on x86
+    - apparently r0 is unavailable in some insns on sh4?
+    - on x86_64, some regs require prefix bytes
+    - types:
+        - byte/short/int that can use parts of a reg on x86
+        - int/long distinction on aarch64
+        - floating point regs
+        - multi-reg types, typically on 32 bit arches:
+            - 64 bit int on i386
+            - 64 bit double on MIPS?
+        - "special-purpose general-purpose" regs
+            - as above, reg for division, looping, etc.
+        - CHERI and pointers
+* floating point
+    - differences across architectures?
+    - calling conventions
+    - alignment, corner cases, etc.
+    - also, see reg alloc