YJIT: Use a Box for branch targets to save memory

We frequently make branches that only have one target but we used to
always allocate space for two branch targets. This patch moves all the
information a branch target has into a struct and refer to them using
Option<Box<BranchTarget>>, this way when the second branch target is not
present it only takes 8 bytes.

Retained heap size on railsbench went from 16.17 MiB to 14.57 MiB, a
ratio of about 1.1.

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