math: improve Atan, Asin and Acos accuracy
math: improve Atan, Asin and Acos accuracy

pkg/math/all_test.go tests Atan (and therefore Asin and Acos) to a
relative accuracy of 4e-16, but the test vector misses values where
the old algorithm was in error by more than that. For example:

x            newError   oldError
0.414215746  1.41e-16  -4.24e-16
0.414216076  1.41e-16  -4.24e-16
0.414217632  1.41e-16  -4.24e-16
0.414218770  1.41e-16  -4.24e-16
0.414225466  0         -5.65e-16
0.414226244  1.41e-16  -4.24e-16
0.414228756  0         -5.65e-16
0.414235089  0         -5.65e-16
0.414237070  0         -5.65e-16

R=rsc, golang-dev
CC=golang-dev
http://codereview.appspot.com/6302093

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

sanjosh/sanjos100-tipc,Triskite/willstone-goclone,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,Triskite/willstone-goclone,mhennings/marcohennings-go,scirelli/scirelli-go,scirelli/scirelli-go,bryanxu/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,webfd/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,mhennings/marcohennings-go,d0f/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,d0f/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,Triskite/willstone-goclone,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,d0f/go-zh,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,bryanxu/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,bryanxu/go-zh