Add specs for load and dump of String with ivars
Add spec for loading marshalled string with custom constructor
Don't use constructor but set class direct for custom strings

Fixes #2554
Fix dump spec for 1.8 mode
Don't check encoding in 1.8 mode
Fix marshalling in 1.8 mode for String
Revert "Disable fibers on >= 10.7. Fixes #815"

This reverts commit e70c097e4f58130efba7b5f845fba9e16ae3c4eb.

This should be enabled again, so for example Process.cpu_times works
properly for current thread timings. The original reason for this change
is a nonexistant reason now.
Fixed compilation on Mavericks broken by revert in b83a17b84.
Add a Time#- spec for String arguments
Add a Time#+ spec for String arguments
Add Numeric#step specs for String arguments
Fix Time#{+,-} and Numeric#step with float-coercible String
Refactor Float coercing methods