Add support for ARM 32 bit

[qarmin]

Fixes #14

Description

This adds support for cross-compiling(or also compiling) app on armv7 - 32 bit.

I tried to find a way to make cross-compiling easier, but not found anything(I wanted to remove libsamba from build dependencies, but looks that for dynamic libraries, also this libraries needs to be available during build)

Tested via cross tool

cargo install cross

added Cross.toml file

[build]
pre-build = [
    "dpkg --add-architecture $CROSS_DEB_ARCH",
    "apt-get update && apt-get install -y libsmbclient-dev:$CROSS_DEB_ARCH",

and then executed

cross build  --target armv7-unknown-linux-gnueabihf -v --example tree --release

Type of change

Please select relevant options.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the contribution guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I formatted the code with cargo fmt
• I checked my code using cargo clippy and reports no warnings
• I have added tests that prove my fix is effective or that my feature works
• I have introduced no new C-bindings
• The changes I've made are Windows, MacOS, UNIX, Linux compatible (or I've handled them using cfg target_os)
• I increased or maintained the code coverage for the project, compared to the previous commit

[qarmin]

@vesso can this be merged and new version released?
My team wants to use this library on arm device, but we can only use versions from crates.io - we want to avoid creating useless forks and publish it on crates.io.