add size limits for unlimited datatypes

[mcb5637]

currently, the size of an integer is only limited by the available memory, which could lead to problems.
add a fixed size limit, generously over the limits required by the specifications.

• integer (including negative, positive and non-negative, non-positive)
• decimal
• also document this properly
• conversion double→integer

XSD required datatype ranges: https://www.w3.org/TR/xmlschema11-2/#partial-implementation

• for Decimal with $i/10^k$, 64-bit integer is enough for $i$ and for $k$ 1 Byte would be enough. ($i < 10^{16} < 2^{63}$)
• Conversion from double to decimal needs at least 1024 bit integer to be lossless.
  • boost fixed size 128 bit integer does not throw exceptions on overflow (most likely fallback to [__int128](https://gcc.gnu.org/onlinedocs/gcc/_005f_005fint128.html))

RDF4CPP should implement:

• arbitrary precision integer types: __int128 and unsigned __int128
• decimals: i: 128 bit, k: 64 bit
• for conversion double→integer: temp 1024 bit integer, throw if 128 bit integer is exceeded

[liss-h]

blocked by BETA