ext/bcmath: Moved macros and added a test

Simplify conversion in BCMath (#14157)

This simplifies the code, and also might indirectly improve performance
due to a decrease in instruction cache pressure. Although the latter is

Simplify conversion in BCMath (#14157)

This simplifies the code, and also might indirectly improve performance
due to a decrease in instruction cache pressure. Although the latter is
probably negligible.

This works because 0x30 has no overlapping bits with [0, 9].

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Use bulk conversion in BCMath of BCD/CHAR where possible (#14103)

On my i7-4790 with benchmark from #14076, on top of #14101 I obtain the
following results:

before (with #14101)

Use bulk conversion in BCMath of BCD/CHAR where possible (#14103)

On my i7-4790 with benchmark from #14076, on top of #14101 I obtain the
following results:

before (with #14101):
```
1.672737121582
2.3618471622467
2.3474779129028
```

after (with #14101 + this):
```
1.5878579616547
2.0568618774414
2.0204811096191
```

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