vm.c (env_alloc): inline to avoid extra zeroing

tiny speedup [ruby-core:64980]

benchmark results:
Execution time (sec)
name    trunk   built
app_lc_fizzbuzz 100.411 98.692

Speedup ratio: compare with the result of `trunk' (greater is better)
name    built
app_lc_fizzbuzz 1.017

rb_proc_alloc changes will give more

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47552 b2dd03c8-39d4-4d8f-98ff-823fe69b080e