Rollup merge of #40709 - lifthrasiir:leaner-unicode-debug-str, r=alexcrichton

Reduce a table used for `Debug` impl of `str`.

This commit shrinks the size of the aforementioned table from 2,102 bytes to 1,197 bytes. This is achieved by an observation that most `u16` entries are common in its upper byte. Specifically:

- `SINGLETONS` now uses two tables, one for (upper byte, lower count) and another for a series of lower bytes. For each upper byte given number of lower bytes are read and compared.

- `NORMAL` now uses a variable length format for the count of "true" codepoints and "false" codepoints (one byte with MSB unset, or two big-endian bytes with the first MSB set).

The code size and relative performance roughly remains same as this commit tries to optimize for both. The new table and algorithm has been verified for the equivalence to older ones.

In my x86-64 macOS laptop with `rustc 1.17.0-nightly (0aeb9c129 2017-03-15)`, `-C opt-level=3 -C lto` gives the following:

* The old routine compiles to 2,102 bytes of data and 416 bytes of code.
* The new routine compiles to 1,197 bytes of data and 448 bytes of code.

Counting a number of all printable Unicode scalar values (128,003, if you wonder) by filtering `0..0x110000` with `std::char::from_u32` and `is_printable` took 50±7ms for both. This can be surprising as the new routine *has* to do more calculations; this is partly explained by the fact that a linear search of `SINGLETONS` has been replaced by *two* linear searches for upper and lower bytes, which greatly reduces the iteration count.

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