Tracking issue for complete 32-bit support

There is a number of small issues making usage of the lib on 32-bit systems (such as embedded chips) impossible or limited. I’ll collate them in this issue with the relevant PRs when I or someone else tackle them, in order to avoid opening many issues but still remember the list.

• 32 bit support on mips and powerpc platforms: was already done by @chitoku-k in #42, thanks !
• 32 bit support on all platform #201 #203
• u32 support for Counter: has been done by @brocaar in #173, thanks !
• f32 support for Counter: #216
• u32 support for Gauge: was already done by @koushiro in #191, thanks !
• f32 support for Gauge: #216
• i32 support for Gauge: has been done by @brocaar in #173, thanks !
• Histograms support for something smaller than f64: this one is not a blocker as Histograms use a RWLock rather than Atomics so it works (non-atomic f64 support is always present in Rust via emulation), but it may be slower or consume more memory than wished for

The remaining work is the unchecked histogram item: support a type smaller than f64 for 32-bit systems. Start by locating the histogram implementation and reviewing the completed 32-bit support work referenced in this issue; done means the smaller representation works correctly on supported 32-bit targets without regressing existing histogram behavior.