* Make the underlying builtins for FP + Vector match Integer.
For stdlib integer types, these are named `_value` and `init(_ _value: Builtin.xxx)`. This patch adopts the same scheme for stdlib floating point and SDK overlay vector types, and removes a legacy init for integers that was only needed to support them. There should be no changes visible outside of the stdlib, and no functional change within the stdlib; the naming of some implementation details is simply more uniform now.
