`signal` improvements

[jorenham]

A "quick" github survey of scipy usage shows that scipy.signal is the second most popular subpackage, and is used twice as often as scipy.sparse.

Currently, many of the scipy.signal functions have suboptimal (broad) return (d)types. These are the ones that showed up in that survey (and their usage counts):

• lfilter (141) - ✨ signal: improved filter function annotations #768
• find_peaks (83) - ✨ signal.find_peaks: specialized return types for all 32 input combinations #945
• butter (78)
• savgol_filter (69)
• filtfilt (66) - ✨ signal: improved filter function annotations #768
• convolve2d (60) - ✨ signal: improved convolution and correlation functions #767
• resample (57) - ✨ signal: improve resample #828
• medfilt (49) - ✨ signal: improved filter function annotations #768
• fftconvolve (40) - ✨ signal: improved convolution and correlation functions #767
• firwin (39)
• welch (31)
• hilbert (27) - ✨ signal: improved filter function annotations #768
• stft (24)
• freqz (23)
• convolve (23) - ✨ signal: improved convolution and correlation functions #767
• argrelextrema (22)
• istft (20)
• get_window (16)
• decimate (14)
• medfilt2d (14) - ✨ signal: improved filter function annotations #768
• correlate2d (14) - ✨ signal: improved convolution and correlation functions #767
• correlate (13) - ✨ signal: improved convolution and correlation functions #767
• sosfilt (10) - ✨ signal: improved filter function annotations #768
• peak_widths (9)
• resample_poly (9) - ✨ signal: improve resample_poly #829
• detrend (8)
• sosfiltfilt (8) - ✨ signal: improved filter function annotations #768
• argrelmax (7)
• iirfilter (6)
• spectrogram (6)
• coherence (6)
• kaiserord (5)
• peak_prominences (5)
• chirp (5)
• cheby1 (5)
• lombscargle (4)
• wiener (4) - ✨ signal: improved filter function annotations #768
• lfilter_zi (4) - ✨ signal: improved filter function annotations #768
• argrelmin (3)
• kaiser_beta (3)
• iirdesign (3)
• place_poles (3)
• iirnotch (3)
• upfirdn (3)
• sawtooth (3)
• cont2discrete (2)
• kaiser_atten (2)
• lsim (2)
• correlation_lags (2)
• zpk2tf (2)
• find_peaks_cwt (2)
• square (2)
• lfiltic (2) - ✨ signal: improved filter function annotations #768
• spline_filter (2)
• cheb1ord (2)
• periodogram (2)
• remez (1)
• minimum_phase (1)
• group_delay (1)
• bessel (1)
• unit_impulse (1)
• buttord (1)
• sosfilt_zi (1)
• normalize (1)
• tf2sos (1)
• zpk2sos (1)
• firwin2 (1)
• oaconvolve (1) - ✨ signal: improved convolution and correlation functions #767
• max_len_seq (1)
• firls (1)
• csd (1)
• gauss_spline (1)
• qspline1d (1)
• cspline1d (1)
• qspline1d_eval (1)
• cspline1d_eval (1)
• qspline2d (1)
• cspline2d (1)