Release 0.9.17 - Rational approximation, indexing, Android fixes

This incremental release delivers new features and fixes a number of bugs.

New features
============

* Add `→Qπ` command for rational approximation with π, √n, ln(n) and e
* Apply `→Q` and `→Qπ` to numbers in expressions
* Add array/list indexing support in algebraic expressions e.g. `'L(1)'`
* Add ability to `Store` / `Copy` to `L(1)`
* Add "real" `ConstantName` and `ConstantValue` commands

Bug fixes
=========

* Fix parentheses around exponents in graph rendering of `'(a+b)^(c+d)'`
* Fix label for `ToRelativeUncertainty`, `→RelRnd` instead of `→RelUnc`
* Fix command alias mapping for `RNrm`
* Improve parsing of implicit multiplication for a number of edge cases
* Fix `utf8_next` for garbage bytes following ASCII characters
* Fix unsatisfied symbol errors with gcc/emcc in the WASM build
* Save Android system state when app is suspended or hidden
* Ensure Android help resources are extracted and available at runtime
* Restore image testing functionality in the test framework
* Updated test images that had become obsolete
* Report errors when saving reference files in tests
* Avoid stack corruption / crash when parsing malformed help index
* Fix computation of `DFC(2.3)`
* Fix null-pointer crash if the multiple equation solver fails

Enhancements
============

* Constrain `→Q` and `→Qπ` behavior to FIX/SCI/ENG setting (like HPs)
* Minor optimization for `program::run`
* Save about 3.4K of RAM on physical devices
* Speed up test runs while increasing default wait time for stability
* Documentation cleanup and additions, add more RPL examples
* Add clear build-time error message when a help topic exceeds 80 characters
* Add demo for interactive menu use
* Optimize away redundant argument checking
* Documentation for DCF and DCF2F

Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>