Moved numeric string conversion functions into own module
Added NumStrConv trait and impls for the string conversion functions
Made num <-> str conversion functions use NumStrConv trait

Removed hacky dependency on Round trait and generic infinity functions
Removed generic-runtime-failure-depending-on-type behavior
Long lines, proper FIXME, inlining
Removed generic infinity, NaN and negative zero functions
Removed Round impl for integers
tidy
Removed `has_*` predicates from NumStrConv trait
Moved `is_*` predicates into standalone functions
auto merge of #4957 : Kimundi/rust/incoming, r=catamorphism

Moved them into own module and made them not depend on an Round trait impl for integers and generic math functions that can fail on integers any more.