Squashed 'evfibers/' changes from 43b7a92..a4ebb2e

a4ebb2e Merge commit 'e15eea2fec7fc4b94a20776b81824492b47e2f1f' into develop
aa01e51 Added fbr_recv wrapper
0ab8102 Changed return type for fbr_eio_custom to be ssize_t
fb91a2f Added fbr_send wrapper

git-subtree-dir: evfibers
git-subtree-split: a4ebb2ef17a26dea32427a41ae4162d37cc69bd4
Merge commit 'db58e6dcfe437a7a125eb464ed89beab018e313c'