@effects is too low a level, and not meant for general usage outside
the standard library. Therefore it deserves to be underscored like
other such attributes.
The main part of this is to rewrite the small string literal-constructor to work with values (= shifting bytes) instead of setting bytes in memory.
This allows the compiler to fold away everything and end up with the optimal code for small string literals.
As this function is generic, it makes a big difference when it can be specialized for concrete sequences, like arrays or unsafe buffers.
This fixes a performance regression of String(decoding:as:), e.g. when constructing a String from a byte buffer.
This adds a small string representation capable of holding up to 15
ASCII code units directly in registers. This is extendable to UTF-8 in
the future.
It is intended to be the preferred representation whenever possible
for String, and is intended to be a String fast-path. Future small
forms may be added in the future (likely off the fast-path).
Small strings are available on 64-bit, where they are most beneficial
and well accomodated by limited address spaces. They are unavailable
on 32-bit, where they are less of a win and would require much more
hackery due to full address spaces.
