Integration of fixed point sub-functions - 1

[x] init_decoder_fx integrated
[x] acelp sub-function: hf_synth block integration
[x] cleanup changes for core_switching_pre_dec_ivas_fx
[x] integration of sub - functions into stereo_tcx_core_dec.
Intermediate conversions to be cleaned up.
[x] acelp sub-function: non_linearity_fx integration
[x] stereo_decoder_tcx_fx updates.